The AM Forum
March 28, 2024, 06:01:11 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1] 2   Go Down
  Print  
Author Topic: I.f. Noise Blankers & Audio Noise Limiters  (Read 23412 times)
0 Members and 1 Guest are viewing this topic.
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« on: April 16, 2007, 12:37:44 PM »

I’ve started a new topic here based on comments from the “Manual for WJ8718 Available” topic, not wanting to continue taking that thread off-topic.

The thread shifted to i.f. noise blankers due to my commenting that the WJ8718 receiver does not have a noise blanker.
- - - - - - - - - - - - - - - - - - - - - - -
Partial quote of  W3JN –

I've found noise blankers and other ham radio toy options to be of limited usefulness,.  Passband tuning might be one exception; the Cubic R-3030 has this (in CW mode only).  The 8716, despite its simplicity and lack of hammy hambone gingerbread, is one stout performer with absolutely fantastic receive audio.
- - - - - - - - - - - - - - - - - - - - - -
De Tom WA3KLR –

John,

I live out in the country and still find the i.f. noise blanker quite important.  My receivers that have i.f. blankers work quite well.  I wouldn't want to be without an i.f. blanker on my main listening receiver.  HF Mobile would be almost impossible without one.  Are you in a noise-free environment or are your blankers not so good?

Years ago I built up a mini-box with a 555 timer running at about 100 Hz. powered by a 9 V battery.  The output is coupled through a few pf. to a 51 Ohm resistor on a BNC jack.  One of the handiest things I have ever built.  Sounds just like engine ignition interference.  It gives the receiver about an S-9 level. 

I couple a wire from an r.f. signal generator by the box and crank up the level until I hear a weak carrier in the receiver.  Turn on the 9 V battery and then the blanker, the receiver should again recover the weak carrier.  All receivers I have tested, do well in this test, or eventually do well in this test after repair and i.f. tuning.
- - - - - - - - - - - - - - - - - - - - -
De Steve WB3HUZ –

What happens when listening to a strong AM signal with the blanker on?
- - - - - - - - - - - - - - - - - - - - -
Sunday, April 15, 2007
De Tom WA3KLR -

Amazingly Steve, I have never noticed a problem with this.  I use a Drake R-7 and Kenwood TS-430 and leave the blankers on all the time. 

No doubt there are receivers out there with blanker problems, especially the ones with the Russian woodpecker type of blanker that greatly increases the blanking time.  They would sometimes trip on SSB signals and add distortion, and in spite of having an adjustable level too.  In that case you would have to resort to having the blanker turned off and bear with the impulse noise.  The Signal One CX-11 is a great example here.  Is that a real transceiver?
- - - - - - - - - - - - - - - - - -
De John W3JN –

Tom, I have NBs in the FT-1000MP mk V, the TR-7, Drake DSR-2, and Collins HF-380.  All I notice is more distortion with no corresponding reduction in noise.  I really have little/no impulse noise here.   ANLs are a bit more useful as they clip the peaks of static crashes.  No modern radio I'm aware of has this feature.
- - - - - - - - - - - -
De Steve WB3HUZ –

The admittedly few receivers I've used with blankers, have always distorted stronger signals, especially AM, and/or produced splatter-like distortion from strong but off frequency signals. I have found them effective in radically reducing impulsive type AC powerline noise, especially when in the SSB (gasp) reception mode.

Unlike JN, I've found very little use for ANL or similarly named circuits on most receivers, for AM use. They seem to be nothing but distortion generators. If static is a problem, I turn down the RF gain or sign off. The NC-303 noise limiter was nice because it was adjustable. You could back it off so it wouldn't distort the audio, but still get some useful limiting out of it.
- - - - - - - - - - - - - - - - - - - - -
De Frank WA1GFZ –

Tom,
I spent the better part of a year trying to build the world's best noise blanker in my homebrew RX. I was never very happy with it. Taking my stock RA6830 compared to the SDR set up the SDR software has a very slight advantage over no noise blanker.
I have found many noise blankers replace noise with the control signal pulse increasing distortion. I have tried many different RF switches and the result was the same. Even if you came close to blanking the noise the silence also caused a transient. The best I ever did was reduce it a bit....and still I wasn't happy. I even had a delay line in the signal path to give me time to process the blanking pulse. Range gating trick.
- - - - - - - - - - - - - - - -

Well I’ve been quite surprised to find out that many receivers have poor i.f. noise blankers or perhaps people are using receivers without noise blankers.

John, you mentioned poor results with the TR-7.  I think I have the same blanker in my R-7 and it is fine.  It is a whole plug-in board called the NB-7, probably something like Frank’s venture at a complicated blanker.  I have not looked at the schematic in many years but I recall it has its own AGC.

So I have 2 decent i.f. blankers in the R-7 and the TS-430.  These are at the opposite end of the complexity scale.  Again I have not looked at the -430 blanker schematic in many years, but I think it is fairly simple.

Over the last couple years of listening, I have come to the opinion that a fair percentage of the hams listening on AM can’t hear worth sh**.  Perhaps the i.f. blanker is a big part of the situation.

John, another thing you mentioned that I have realized also is the lack of an audio noise limiter in receivers the last 30 years.  Many of the old tube receivers had an audio noise limiter of some kind and most were very effective.   Unfortunately though, many distorted the audio also which is not necessary – an overly tight clipping level.  Perhaps this contributed to it’s later disuse.

When the receivers started coming out with the i.f. blankers, primarily since the solid-state transceivers appeared I think, the ANL was abandoned.  But while the i.f. blanker is very effective in eliminating impulse noise, it can’t eliminate the a.c. line noise which is a wide audio pulse about 0.5 milliseconds coming every half cycle at 60 Hertz.

The audio noise limiter and i.f blanker are both needed.  Long before this thread, I have been planning to build some home-brew AM receivers and I have been looking at the receiver audio output with line noise present.

So we have had this period of over 25 years of receivers with an i.f. blanker but no audio noise limiter!  The latest receivers with DSP are addressing this, it looks like.  In the ads, I see on the front panels of the new transceivers, a button usually labeled “NR” for noise reduction.  I presume that there is some sort of DSP audio noise reduction, a terrific application for the DSP.  But you can’t beat the performance of just a simple good audio clipper added to the receiver audio.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
w3jn
Johnny Novice
Administrator
Member

Offline Offline

Posts: 4620



« Reply #1 on: April 16, 2007, 12:55:35 PM »

One reason nobody puts ANLs in receivers anymore is they aren't effective with a product detector and slow decay AGC.  The mfgrs are catering to the slopbucket/CW market.

Yes, ANLs add a bit of distortion but a well-designed ANL enhances, rather than detracts, in cases of big static crashes or harsh buzzies.

In my experience noise blankers work very well for very short duration impulses but are worthless for lightning static, light dimmer buzzies, TV horizontal buzzies, etc.  Many are also poorly designed - they add intermodulation distortion, mixing products, etc.  It appears that (I haven't verified this) most of the ones in the rigs I have are poorly engineered in that they detect noise pulses AFTER the crapstal filtering.  The filter elongates the pulse, shifts its phase all over the place, and the result is poor NB action.

IMHO the best way to design a NB is to do like Squires-Sanders did in the SS-100.  That is, have a whole separate wideband IF strip hanging right off the first mixer and blank the pulse before it even hits the crapstal filter.  The Collins NBs that were accessories in the KWM-2 might be another example (Never used one) - it had IIRC a 40 MHz IF amp but I don't know where the blanking action actually was inserted.
Logged

FCC:  "The record is devoid of a demonstrated nexus between Morse code proficiency and on-the-air conduct."
W2VW
Contributing
Member
*
Offline Offline

Posts: 3489


WWW
« Reply #2 on: April 16, 2007, 01:25:03 PM »

Some noise blankers seem to do nothing. Here, they make the difference between being able to get on the air or not. My older receivers sit and gather dust. Noise here is from very poorly maintained power lines and close in oil burner type heat. Some CB rigs have the blanker with separate R.F. noise amp strip. 2 best blankers I've used: TS-440 and Sears Roadtalker 40 channel. Obviously the roadtalker isn't being used too much as 10 is pretty dead. A good project would be to crack open an old CB and swipe the noise blanker. Time to move to the woods so I can walk by the windows with no clothes on and use boatanchor receivers.
Logged
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 8154


CQ CQ CONTEST


WWW
« Reply #3 on: April 16, 2007, 02:09:44 PM »

IMHO the best way to design a NB is to do like Squires-Sanders did in the SS-100.  That is, have a whole separate wideband IF strip hanging right off the first mixer and blank the pulse before it even hits the crapstal filter.  The Collins NBs that were accessories in the KWM-2 might be another example (Never used one) - it had IIRC a 40 MHz IF amp but I don't know where the blanking action actually was inserted.

The Squires Sanders receivers SS-1R and SS-1BS (no model called SS-100) noise blanker(SS-1S) is very effective for killing pulse type noise. Kenwood patterned a similar design in many of their 70's rigs and, I believe (don't have the schematics in front of me) Drake also patterned a similar type of noise blanker. Hammarlund offered an adjunct type assessory to their HQ-170 and 180 receivers called an IF Noise Silencer to blank pulse noise. Don't remember it being highly effective.
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
The Slab Bacon
Member

Offline Offline

Posts: 3934



« Reply #4 on: April 16, 2007, 02:22:46 PM »

Dave,
       the most incredible noise blanker / liniter I have ever seen was in a cb radio. I had a Royce 23 channel rig 30 years ago. This thing had a button simply labeled "ANL". It eliminated all of the ignition noise completely without any distortion to the audio at all! Absolutely none.

I was always impressed with the performance of that ANL, and the lack of audio distortion, even on loud strapping signals. The pulse type noise was COMPLETELY ELIMINATED!! I was never able at that time to get a skizmatic on the rig. I really wanted to build whatever they used for their ANL into a few other radios.

                                               The Slab Bacon
Logged

"No is not an answer and failure is not an option!"
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #5 on: April 16, 2007, 02:27:49 PM »

I have a TR7A with NB7 not really impressed with it.
I found the best noise blanker for the wood pecker was the F117A.
Logged
W2VW
Contributing
Member
*
Offline Offline

Posts: 3489


WWW
« Reply #6 on: April 16, 2007, 02:36:31 PM »

Dave,
       the most incredible noise blanker / liniter I have ever seen was in a cb radio. I had a Royce 23 channel rig 30 years ago. This thing had a button simply labeled "ANL". It eliminated all of the ignition noise completely without any distortion to the audio at all! Absolutely none.

I was always impressed with the performance of that ANL, and the lack of audio distortion, even on loud strapping signals. The pulse type noise was COMPLETELY ELIMINATED!! I was never able at that time to get a skizmatic on the rig. I really wanted to build whatever they used for their ANL into a few other radios.

                                               The Slab Bacon

That is just how the Sears one behaves. Strong signals don't distort. The TS-440 sure does. This last estate pile here has a bunch of junk CB sets.Time to start peering at the innards.
Logged
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #7 on: April 16, 2007, 04:11:28 PM »

I later found the shunt switch blankers worked better than the series switch. The shunt justs loads the signal down so it was more effective.
I was trying to be too smart with mine so I crashed and burned.
Logged
Steve - WB3HUZ
Guest
« Reply #8 on: April 16, 2007, 04:25:23 PM »

Great topic. Thanks for redirecting and compiling Tom.

For locally generated powerline noise, I prefer using antenna phasing/noise canceling. This keeps the offending noise from ever entering the receiver. Another option is a loop to null out the noise. Even if you have a relatively quiet location, the key to really "hearing" well is to have one or more specialized receive antennas.
Logged
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #9 on: April 16, 2007, 05:20:10 PM »

Many people are doing well with the MFJ Noise Canceller - can't name the model off the top of my head.

The CB blanker – In 1977 I bought a 23 channel side-band Midland transceiver when they were being sold out as the band went to 40 channels.  I immediately “converted” the rig to 20 meters with a lot of additional overhaul that wasn’t originally intended, like a ‘GFZ project I guess, it really grew. 

Circuit description -
The blanker input hangs on the r.f. stage TR1 collector.  The signal is immediately rectified by D5,D6 (looks like a doubler) and is amplified by a JFET (FET1).  Output of the JFET is amplified by PNP (TR3).  Output of TR3 drives NPN (TR4) whose collector keys a 0.005u cap to ground.  The other end of the cap hangs on the hot end of the mixer (TR2) output coil and bypasses the mixer output to ground when TR4 conducts.  Also this mixer output coil node gets rectified by D8,D9 and this voltage is put at the bottom end of the r.f. doubler detector D5,D6 as a reference and forms feedback.  The feedback would set the operating point and probably allows the blanking pulse to self-complete.  Runs off of +8 Volts.

I never hooked the circuit back in to the 20 meter transceiver.  So I can’t comment on it’s operation.  But odds are the radio Dave is referring to may be very similar.

Note that the blanker input picks off with only the 2 r.f. coils as selectivity ahead of it and blanks before the i.f. filter.  This radio used a compromise i.f. filter bandwidth of 4 kHz. for the AM/SSB rig.

Below is a rendition of the schematic area that is relevant:


* Midland_cb_nb2.JPG (139.25 KB, 1500x860 - viewed 929 times.)
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
W2VW
Contributing
Member
*
Offline Offline

Posts: 3489


WWW
« Reply #10 on: April 16, 2007, 05:38:26 PM »

Tom, you should write the "Big Book Of Radio Circuits Explained." The part about the feedback setting the operating point would have totally escaped me. Excellent. 
Logged
w3jn
Johnny Novice
Administrator
Member

Offline Offline

Posts: 4620



« Reply #11 on: April 16, 2007, 06:46:13 PM »

Pete as usual you are correct - SS1R.  Squires-sanders used a transistor hanging between the output of the mixer and ground.  The wideband IF would rectify noise pulses and drive the base, tjereby shunting the mixer to ground just for the duration of the noise pulse.  Because the IF was so wide (a MHz IIRC) the noise pulses weren't shaped and care was taken to prevent phase shifting.

I forgot another receiver I have with a NB, and that's the SX-28.  It has a Lamb-style "noise silencer" using a 6L7 and a noise amp tube, IIRC.  I don't remember being too impressed with its effectiveness.

Moving into the woods, Dave, might be a good thing, but would your wife really appreciate you hanging (so to speak) around the house in such a state?  Grin
Logged

FCC:  "The record is devoid of a demonstrated nexus between Morse code proficiency and on-the-air conduct."
W2VW
Contributing
Member
*
Offline Offline

Posts: 3489


WWW
« Reply #12 on: April 16, 2007, 07:30:17 PM »


Moving into the woods, Dave, might be a good thing, but would your wife really appreciate you hanging (so to speak) around the house in such a state?  Grin

Might be fun but the ruts in the carpet would get costly.
Logged
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #13 on: April 16, 2007, 07:53:56 PM »

Thanks Dave.  No book though.

At one time, Squires-Sanders was down the road from Irb's QTH.  They moved around to a number of work sites in the area.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 8154


CQ CQ CONTEST


WWW
« Reply #14 on: April 16, 2007, 09:02:40 PM »

Thanks Dave.  No book though.

At one time, Squires-Sanders was down the road from Irb's QTH.  They moved around to a number of work sites in the area.

Actually, as far as I can tell, they were only in three places in NJ. The original Clegg location at Route 52, Mt. Tabor; Watchung Ave., Watchung (only for a very short period); and the final location at Martinsville Road/Liberty Corner, Millington. This last place is close to Irb's location. After Squires Sanders vacated this building, I believe Honeywell took over the building. Neither Mt. Tabor or Watchung is very near Irb's location.
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
AF9J
Guest
« Reply #15 on: April 16, 2007, 09:33:09 PM »

Hi Tom,

The unit in question is the MFJ-1026 (which comes with a whip antenna), and the MFJ-1025 (which comes without the whip antenna). They cost about $150-$170. Timewave also bought the rights to the JRC antenna noise cancelling system.  It's marketed as the ANC-4, and costs about $200. The Timewave unit is considered the better of the two systems.  My noise cancelling system is the ANC-4.  Once I realized that I was trying to cancel out atmospsheric noise (duh!), I was able to get rid of my powerline noise on 75m, and actually receive AM for a change.  BUT, you do have to do a fair amount of tweaking to cancel the noise.  Also, from my experience, the noise canceling effect is not necessarily for the entire band.  And, since I live less than a mile from two AM broadcast stations, I had to activate a built-in high pass filter, to keep from amplifying the broadcast signals, to the point where they blow through to my receiver.  As it is, I still have some broadcast blowby.  There's an internal notch filter in my ANC-4 that will handle this, but it only does it in 3 bands, the upper third, middle third, and lower third of the AM broadcast band.  So if each station is in a different portion of the band, I'm out of luck.  Also, the whip antennas are only for noise, in the house.  For powerline noise, you have to use an outside antenna.  Still, so far, it has helped my reception.

73,
Ellen - AF9J

Many people are doing well with the MFJ Noise Canceller - can't name the model off the top of my head.

Logged
Ian VK3KRI
Guest
« Reply #16 on: April 17, 2007, 07:24:26 AM »


De Frank WA1GFZ –

Tom,
I spent the better part of a year trying to build the world's best noise blanker in my homebrew RX. I was never very happy with it. Taking my stock RA6830 compared to the SDR set up the SDR software has a very slight advantage over no noise blanker.
I have found many noise blankers replace noise with the control signal pulse increasing distortion. I have tried many different RF switches and the result was the same. Even if you came close to blanking the noise the silence also caused a transient. The best I ever did was reduce it a bit....and still I wasn't happy. I even had a delay line in the signal path to give me time to process the blanking pulse. Range gating trick.
- - - - - - - - - - - - - - - -

I tried building a blanker using two receivers. One to pick off pre filter IF signal to get the pulses, then a couple of PIN diodes to switch out RF into the 2nd receiver.  The idea being that reciever 1 can be tuned to a part of the band with lost of pulse noisw , but no carrier, and the 2nd tuned to the signal of interest.

This was a complete failure because, as Frank mentions, chopping a hole in an AM signal results in  effectivly a  square wave pulse of negative 100% modulation (no signal). This was considerably more objectionable than the original pulses.  Narrowing the the 'hole' might be a solution, but audio or IF limiting rather than blanking sounds like it may work better - and only require one receiver!

   Ian VK3KRI   
Logged
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #17 on: April 17, 2007, 08:42:04 AM »

I built an RF switch and the control signal leakage was too strong at about -80 db. I used balanced transformers and matched diodes and that hole was worse than the noise. I even did pulse shaping but still couldn't get it right. Steve QIX's 813 PDM rig would fire it all the time.
It made me very humble.
Logged
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #18 on: April 17, 2007, 01:38:45 PM »

Hi Ian,

Thanks for bringing up the technique of i.f. limiting, you jogged my memory.  That reminds me of my Sony portable receivers.  I have a ICF2001 and a ICF2010.  Perusing the schematic one time I noted that they run the AM receive path through the FM chip, I don’t recall which schematic this was.  It seemed that the receiver did have less impulse noise output.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #19 on: April 17, 2007, 02:32:23 PM »

I've also seen a method that gated off the LO to a mixer. I think it was a Rohde article from the cobwebs of my mind. I never tried it.
I took the first IF and split it to two channels. Main got a delay line and other got the pulse detection and blanking shaping. I tried a number of methods and each time I came up with a bright idea I was shot down in flames. The more complicated it got the worse it worked.
I even went so far to down convert the detection which really screwed me.
Logged
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #20 on: April 17, 2007, 04:31:40 PM »

Frank now you have jogged my memory on an i.f. blanking technique I saw where the 2nd l.o. was pushed way off frequency for the blanking pulse, so that the i.f. selectivity would provide the attenuation for the blanking.  Has merit, no charge injection.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
WA1GFZ
Member

Offline Offline

Posts: 11152



« Reply #21 on: April 17, 2007, 05:11:03 PM »

Gee that is an interesting idea. My homebrew RX has a 50 mHz oscillator for the second mixer. Just modify it to a VCXO.
But then here is the next problem. Noise is Broadband so you swing the frequency and the signal you want hear goes away but the BB noise is still there.
I lost way too much sleep on this subject......I think it is a good one for the DSP guys.
Logged
Bacon, WA3WDR
Member

Offline Offline

Posts: 881



« Reply #22 on: April 17, 2007, 07:34:38 PM »

I remember a GE noise blanker for 30 to 50 MHz commercial FM work from the Mastr-II days. It had a wide, wide receiver, many MHz wide, centered on the operating frequency, and RF in this range was amplified and detected, and that was used to mute the received signal somewhere, I don't remember the details of the circuit.  But it had a 27 MHz notch, so CBers wouldn't mute the receivers.

For 60 Hz AC line noise, I tried applying 60 Hz to the gate of a j-fet, and using the fet to short the receive input at a relatively high-Z point. With the right bias and drive, this reduced the buzz nicely, but it caused 60 Hz intermodulation.

Now I'm thinking of a relatively wideband pulse detector and a muting system that substitutes a 'fill' signal based on recent received signal.  This would be swamped by long lighting crashes, but for a lot of pop-pop and light crackle pulses it would be great.
Logged

Truth can be stranger than fiction.  But fiction can be pretty strange, too!
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #23 on: April 17, 2007, 10:53:10 PM »

John, referring to a comment of yours way back in relation to impulse and line noise interference and product detectors, yes I noticed that the audio output is a little different with product detectors as compared to the receivers with diode detectors.  With the usual diode detectors which is a half-wave rectifier, energy bursts usually wind up to be polarized in the same direction, no surprise.

I noticed that the R-7 receiver with the product detector showed recovered audio with line noise pulses of bi-polar content but overall still showed stronger spikes in one direction, as I recall.  I don’t see how slow AGC would make the spikes less liable for clipping.  I believe that an ANL will still be effective with product detectors.

TS-430 and TS-440 noise blankers:

Dave and I both like our -430/-440 i.f. blankers.  I compared the schematics and the -430 and -440 blanker circuits are identical except some different transistor types are used in the -440. 

The blanker circuit input picks off at the 2nd mixer output coil.  So this is after the 20 – 25 kHz. wide roofing filter in the 1st i.f.  The first amplifier in the blanker is a JFET.  This drives 2 NPN’s in a differential amplifier configuration.  The diff-amp output is a resonant transformer which drives an NPN amplifier, resonant-transformer-coupled to a diode detector.  The detector output drives an NPN transistor which keys the bias to a push-pull diode gate following the blanker pick-off point.  Two diodes are in series in each side of the balanced gate for increased attenuation when gated off.  There is a 47 pf. cap. to ground between the 2 diodes to help increase the attenuation.  The diode gate must be similar to what Frank home-brewed.  The idea is to get common-mode rejection by the balanced circuit to minimize blanking pulse injection out of the gate circuit to the following i.f.   There is AGC feedback from the detector voltage of the blanker back to the differential amplifier bias.

The gate output is a tuned circuit which then leaves the RF board for the IF board where the cable runs into another tuned circuit and the switched i.f. crystal filters are next in line.

So these Kenwood blankers and the CB blanker have about the same amount of resources.  Kenwood picks off farther down the line and uses a series gate, where the Midland design used a shunt killer.  Both series and shunt gates immediately precede the i.f. filters.

Bacon, I thought about using an a.c. line triggered audio blanker a few weeks ago, but abandoned the idea after my audio line noise study.  The line noise I saw triggered by the power line occurs near the peak of each half cycle but is very jittery in position.  Now if the line noise was from a lamp dimmer, a problem I don't have here, then the line-triggered blanking may work well because the position of the dimmer's triac turn on is probably very stable.  What I see here is occurring from something other than dimmers - utility line breakdowns or other appliances hanging on the line.  For your audio blanker, you want to be able to adjust the phase delay and have the blanking pulse at least 0.5 millisecond but no longer than 1 millisecond.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 2120



« Reply #24 on: April 18, 2007, 12:37:29 PM »

I took a look at my Drake R-7 manual and NB-7A blanker board schematic this morning.  The blanker is placed in the receiver line-up at the same point as the TS-430 and -440.  That is, in the 2nd i.f. after the 2nd mixer and before the various selectable i.f filters.  The NB-7 board was an option.  Once installed, the signal path then flows though this board.  It is used in the TR-7 also.  (The board became standard in the receivers produced later, being called the R-7A, no design changes, just that some previous options came already installed.  I think a 500 Hz. filter was included too.)

One surprise here is that there is a filter crystal after the input on the blanker board.  This adds another 2-pole filter at the 2nd i.f. in addition to the roofing filters in the 1st i.f.  The series gate in the signal path is a 2-diode balanced gate.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
Pages: [1] 2   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.085 seconds with 18 queries.