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VFOs for broadcast TX's. What are you using?




 
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Author Topic: VFOs for broadcast TX's. What are you using?  (Read 7774 times)
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VE7 Kilohertz
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« on: December 30, 2004, 10:41:06 AM »

Merry Christmas and happy new year to everyone!

Several us of are working on installing VFO's in our bcast rigs and moving away from rock bound operation and I am curious to know what you are using for a VFO and how you interfaced it with your particular rig and what modifications to the transmitter you made to get it working. I have several ideas about how to do it but would like to see what other creative minds are doing.

Look forward to hearing from all you broadcast TX VFO users.

Cheers

Paul
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« Reply #1 on: December 30, 2004, 10:53:21 AM »

I use a Globe 755a self contained unit. On the Collins 21E.  I don't recall how I injected the signal but I beleive it was right into the Cyrstal injection point.

Sweet and simple

Works well
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« Reply #2 on: December 30, 2004, 11:06:59 AM »

I started out using a Johnson 122 VFO with my Gates BC-1G. I injected it into one of the xtal ports - I think I made a slight change to the oscillator unit in the TX - pulled a cap out and changed the value of another - got it in my notes somewhere.

Now I am using an old Fluke digital signal generator. It will allow me to use a long piece of RG-62 and place the VFO near the operatiing position.
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« Reply #3 on: December 30, 2004, 11:07:56 AM »

I use a Kenwood 870 as my exciter/VFO/receiver for the Raytheon RA250. The 813 driver was reconfigured grounded grid/broadband input.  The 870 has an automatic antenna tuner, so input tuning is simple.  It takes 35 watts drive to achieve proper parameters.
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w3jn
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« Reply #4 on: December 30, 2004, 11:22:53 AM »

AN outstanding synthesized VFO can be had for almost nothing in the form of an old HP synthesized signal source.  These beauties go from 1 millihertz up to 13 MHz and have a tracking 13-23 MHz output and have adjustable attenuators that go from -88 to +13 dBm.  I've seen them for as little as $10 (I paid $20 for mine).  The downside is they're a bit big, and you have to JS up a matching network for their 50 ohm output.  They have OUTSTANDING phase noise properties, and you can use them as a low-distortion audio source or as a regular sig gen for aligning receivers, etc.  Their long term stability is 10-9, and short term (after warm up) is comparable.  Much better in all respects than a typical ricebox.  I gave mine to the HUZman who is going to use it with his BC xmitter.

Another beauty is the HP 3330 - like above, but freq input/attenuator setting is via keyboard, and sweeps as well! (either freq or amplitude).  I scored one of these at Timonium for $15 - works perfectly but the front panel paint is all flaked off.  This mates with the 3571A spectrum analyzer (available on eBay for <$100, which is where i got mine) or the 3572A network analyzer.  All OUTSTANDING instruments (if a bit limited in freq range).

73 John
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« Reply #5 on: December 30, 2004, 11:51:03 AM »

Hi Paul,

I use an HP 3320B Frequency Synthesizer on the 20V-3. It is injected through the crystal socket . I used a 50 ohm resistor to match the impedance and a cap to protect the synthesizer since it's output is DC coupled.

Max output is +27 dBm and range is DC to 13 MHz. I use a single pole 3 throw BNC switch. On receive the synthesizer play's into a load as to not fill the shack with RF. On transmit the generator is switched to the crystal socket a moment before the V-3 is keyed. I mounted a toggle switch on this RF switcher and one of the ports has an antenna. I use this position for spotting.

The drawback on the 3320B is a group of vacuum reed switches. They age poorly and aren't available for replacement. If you have a bad one you can lose drive unexpectedly. Hard on the tubes. I'm looking for a more modern unit now. 73

Mike
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WA1GFZ
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« Reply #6 on: December 30, 2004, 11:53:35 AM »

The HP8640B is also a good source that can go into lock mode for a nice clean source.  Good for RF testing also since it has low phase noise.

Also the T195 Collins PTO is a good one that covers 1.5 to 3 MHZ.  fc
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« Reply #7 on: December 30, 2004, 11:53:53 AM »

Here is the Fluke that I am using with 3 transmitters. These units go up to 11 Mhz and are not too big in size (except they are fairly deep). The output is 50 ohms and it works FB with 93 ohm lo-cap coax. One thing you will have to do if using a signal generator of any type, is to find a way to disable, mute, whatever you call it, when not in xmit mode, as the signal is audible in the station RX. I don't have a manual for this one, so I just went in and poked around until I found a module that killed the output when the +22 volt line was removed. I installed a small relay and I key it with control line going to the receiver mute box.

With this generator, the Gates takes 1 volt, the Meissner 3.4 and the PP HB triode rig 1.5volts.

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VE7 Kilohertz
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« Reply #8 on: December 30, 2004, 01:06:30 PM »

Wow. Nice mix of different ideas. I like the generators that you have come up with.

I forgot to ask, what is the osc tube in your particular rig and are you just plugging into the xstal socket? Any load termination or tuning i.e. tank cct at the rig end of the coax?

Cheers

Paul
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« Reply #9 on: December 30, 2004, 10:10:04 PM »

For my BC1-T, I modified the xtal oscillator as follows: (1) ground the grid in the 12BY7 xtal oscillator stage.  (2) mount a female BNC connector on the back of the oscillator unit.  Connect the BNC to the cathode of the 12BY7 using a mica coupling cap.  I  think I used .001 mfd for mine.  It isn't critical.  Keep the rf choke in the circuit.  Disconnect the xtal feedbark components.  (3) Connect low-Z output from the VFO to the BNC.  The 12BY7 acts as a broadband grounded grid amp.  I removed the xtal frequency trimmers from the oscillator unit and moved the tank capacitor that resonates the slug tuned coil to the mounting hole of one of the trimmer caps.  That way you have access to the tank circuit from the front panel, without having to remove the cover to the oscillator unit.  I had to  remove some turns from the slug tuned coil to get it to resonate on 160m even though the manual says the frequency range goes up to 2.0 mc/s.

My low-Z vfo is a T-368 master oscillator unit with an extra wafer added to the bandswitch.  I wound a coupling coil on the cold end of each output coil and used the added wafer to select which coupling coil goes to the output connector.  With some realignment I can get uniform rf output across any amateur band, even though I lost some tracking beyond the amateur bands on frequencies I don't use anyway.

So I have 3 tuned circuits to resonate:  the 12BY7 stage, the driver plate/PA grid stage and final tank circuit.
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« Reply #10 on: December 31, 2004, 10:11:43 AM »

Paul,

The Oscillator in the 20V-3 is a 6AU6. Aside from terminating the 50 ohm output of the frequency synthesizer into a 50 ohm resister and adding a .005 mfd cap in series to protect the synthesizer (all done inside an old crystal can) the original circuit is intact. The only thing needing adjustment was the crystal trimmer. I run the signal input at +5 dBm. Good Luck

Mike
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Philip, AB9IL
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« Reply #11 on: January 16, 2005, 04:08:33 PM »

Paul,

Here's another option - a DDS VFO kit popular with the QRP operators, that can operate from near DC though 6 meters.  It is probably not a cost effective way to eliminate the need for crystals, but it performs well, is small, and uses little power:

     http://www.sseng.com/catalog/dvfo-ii.htm

Those HP signal generators seem to produce a good clean and stable signal and require less effort to interface.  Good luck doing the mod; hope to hear it on the air sometime!
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