Initial Receiver Tests - Mercury HPSDR vs: FT-1000D

[K1JJ]

I ran some preliminary receiver tests using the HPSDR Mercury receiver against the FT-1000D. We all know the filter selectivity, noise blanker functions and dynamic range should be better on the SDR.

But what surprised me is * in the clear * the HPSDR could hear signals that were weaker than the 1000D could copy due to what I think is 1000D phase noise.  This is more pronounced on 20M and higher where atmospheric noise is lower, so the RX noise floor becomes more important.  On 40M and lower, the atmospheric noise usually masked this difference in performance.

I tried both on-air signals and using a signal generator. On 20M and above, the HPSDR front end preamp made a huge positive difference. Using a signal generator, a 14 mhz signal that was barely heard in the noise on the FT-1000D, I could hear clearly on the Mercury because its noise floor was much lower.  I reduced this signal lower, could hear it on the Mercury, but totally disappeared on the 1000D.  The 1000D seems to have cyclic areas throughout the spectrum where the phase noise is higher than other areas. To be fair, I was using a spot with the lowest 1000D phase noise.

Today I tuned in some Russian DX stations on 20M with the stacked 20M Yagis (using a real noise floor) and the weaker ones were easily better copy on the Mercury.

Considering SDR is still in its infancy, the next years of device improvement should leave the older receivers in the dust.


Rich/ETP was telling me about tests he and Chuck ran on 15M with similar results. Now I understand what they were seeing.

T

[Steve - K4HX]

Interesting. Thanks for posting. No knock on the SDR, but I bet you would see the same or similar differences between the FT1000 and any higher-end new receiver, SDR or otherwise.

[K1JJ]

Yeah, sounds logical. The newer FTDX5000MP? that goes for $8K ranks way up the list in the Sherwood tests.  It appears as good as an SDR.

Keeping within a budget, I was always biased to think the 1000D was the best I could do here. It probably was optimum on 160-40M with the higher band noise, but now that low noise 20M-6M are opening up, this difference becomes significant if ya operate there looking for the real weak ones.


T

[Steve - K4HX]

In my short time with the 1000, I've noticed that it is a bit nosier than some newer receivers I've used. It also exhibits some spurs here and there. To be fair, I should continue the comparison after a full alignment of the 1000.

[Pete, WA2CWA]

Yeah, sounds logical. The newer FTDX5000MP? that goes for $8K ranks way up the list in the Sherwood tests.  It appears as good as an SDR.
T

The $8K is List; you can buy one for a mere $6.3K

[K1JJ]

Pete, if I don't eat for 6 months I can afford one at $6.3K.  It might be worth weighing 70 pounds to hear the weak Pascals of the world above 7 mhz.


HUZ, when stick your diddle wand in that 1000D just remember me - I can use another  backup for this transplanted 1000D amplifier...

T

[KF1Z]

Pete, if I don't eat for 6 months I can afford one at $6.3K.  It might be worth weighing 70 pounds to hear the weak Pascals of the world above 7 mhz.

Yup....

I can feed my family of 4 on less than $5k for a whole year. And that's eating pretty well.

My priorities lie elswhere than dumping that wad on one little radio.

 

[WA1GFZ]

Time to do more testing with the Racal and Cubic. Maybe I should drag out the old Heathkit SB303 with the full race cam. I wonder if you checked it against an FT102 that does not have a synthesizer.
The Cubic R303 has an Rf amplifier and input filter bans. It is very sensitive but has more close in phase noise than the Racal.
Mercury should be even cleaner since Flex uses a DDS source which makes close in spurs. Mercury has a crystal oscillator. Nothing is cleaner than a good crystal oscillator. The only hole is the FPGA code doing digital down conversion.
I have done a lot of testing on 80M and the Racal holds its own against Mercury. Mercury has ever so slight agvantage when there is high static.
Racal beat my copy of the Flex front end  hanging off the IF.
Receiver drag racing, I love the smell of two warm HP8640Bs driving receivers to their red line.

[AB2EZ]

Question:

If you put a preamplifier (perhaps with a roofing filter to block strong, off-frequency signals) in front of a receiver, such as an FT-1000D's receiver... or any other decent receiver... and if the preamplifier uses the latest technology... why wouldn't the minimum "detectable" signal (on frequencies where atmospheric noise is lower than the thermal noise of the receiver's front end) be completely determined by the equivalent input noise of the preamplfier?

Stu

[WA1GFZ]

Tom,
Just for the heck of it listen on 10 or 15 and pump your signal generator into the RX. Crank up the level until you hear noise change. Now slowly tune across the bands to see if you have any responses. An OK RX -30 to -20 dBM, a good RX -20, Kick butt 100 db above the noise floor of the RX.
I agree with Syu a good preselector with some gain will help. Harris, Cubic, McKay, Collins etc made some cool prepost seletors made for that putpose.
I need to put mine in service. I would love to be smart enough to have HPSDR control it. It tunes in 10 KHz steps.
Also another cool box that you might like is the Racal dual preselector with 13 bandpass filters .5 to 30 MHz. This would be perfect for diversity. I landed a new one but they come up on ebay once in a while for around $100. In a 3 inch rack. My tunable preselector is in a 1U rack MSR6300. Both have a little gain.
I have a collins unit that has a 4 gang cap as the tuner and full of vacuum relays to select inductors

[K1JJ]

Sure, I'd like to try a good quality preselector for the Mercury.  I don't mind manually tuning it either if it will show better results.  Let's say I don't have any images or spurs - just what will the preselector enhance otherwise?



"An OK RX -30 to -20 dBM, a good RX -20, Kick butt 100 db above the noise floor of the RX. "

Please explain what you mean here ??


T

[WA1GFZ]

This will tell you where the RX responds to out of band signals or overloads. If you had 2 generators you could do dynamic range testing. Another thing I try is use a hybrid combiner with the antenna on one port and the generator on the other input. Listen to a weak signal while you tune the generator around to see what it takes to trash the signal you want to hear.  A preselector on mercury would limit the input bandwidth and will filter out some of the broadband noise.

[Steve - K4HX]

A preselector and a preamp will do nothing for internal spurs, birdies, oscillator phase noise and filter leakage.

By adding the preamp you will likely reduce the dynamic range of the receiver.

The FT-1000 has a preamp. It is on when the Front End control is set to the Normal position. It is out when this control is set to IPO position.

[w3jn]

One advantage the HPSDR has is there's no LO, so no phase noise from the LO.  Most likely the clock from the A/D converter might contribute some to noise in the HPSDR however.  Nothing is noiseless 

Phase noise from the LO will modulate the signal and can't be overcome by preselectors or preamps.  In some receivers the phase noise is so bad the hiss is apparent even on strong signals.  I'd bet that if you drove the Yaesu with a couple of HP-8640s it'd be roughly equivalent to the HPSDR.

Last, and this is purely conjecture on my part due to unfamiliarity with DSP algorithms, but perhaps another factor is the demodulator code and/or the I/Q nature of the analog input in a SDR might tend to minimize demodulated phase noise.

[AB2EZ]

All: thank you for your comments.

Clarifying my earlier post:

The original post in this thread is focused on weak signal performance. The observation is that the HPSDR receiver can "hear" weaker signals than the Yaesu FT-1000D can hear.

If one uses a preamplifier that employs the same transistor technology as is used in the HPSDR, and if one incorporates a preselector to avoid dynamic range problems caused by strong off-frequency signals (if needed)... then one should be able to

a) amplify the incoming signal and the associated incoming noise by (for example) 10dB

b) achieve the same ratio of incoming signal-to-(incoming noise + preamplfier noise) at the output of the preamplifier as is achieved at the output of the HPSDR's first stage front end.

Once you have added the low noise preamplifier (presumably having somewhat lower preamplifier noise than any of the preamplifiers that are in the circa 1989 generation FT-1000D), you no longer have a "weak" signal coming into the FT-1000D.

It is reasonable to assume that all "birdies", mixer-induced phase noises, A/D conversion artifacts, etc. are introduced after the front end preamplifier... and would be present with both weak and strong incoming signals.

Therefore, if there is a difference in performance of the two receivers that is observable only with weak incoming signals... you should be able to overcome that with the use of a preamplfier employing the latest technology... as is employed in the front end stage of the HPSDR.

Stu

P.S. The reason that modern technology can enable the construction of a preamplifier with somewhat lower noise (maybe a fraction of the decibel or a decibel) vs. a 1989 design is that modern transistors have smaller feature sizes, and, in some cases, higher electron mobility... that yield higher gain-bandwidth products. This allows the use of low noise transimpedance amplifier designs whose noise figures (when driven by a 50 Ohm source signal having the equivalent noise of a 50 Ohm resistor at room temperature) are closer to 0dB. I don't know what the noise figure of the lowest noise FT-1000D preamplifer is at 30MHz ... but I would guess it is around 2dB. It is likely that a design incorporating the latest transistor technology would have a noise figure of less than 1dB.

[WA1GFZ]

My 1975 SB303 could hear down to -140 dBM with a tunable preselector. But it only had 85 dB of dynamic range after a lot of modification. Stock it was 67 dB. It took me another 15 years to build stuff in the mid 90s. Jay and I go back about 35 years in that game.
HUZ very easy to buld a high dynamic range preselector. My cubic will handle 1/2 watt. There is a guy in UK selling high dynamic range tunable preselectors
The point of the preselector is to limit the crud getting into the RX chain.
Yes the DDC function of the FPGA will introduce some distortion but in the case of HPSDR check the test data. It is not an I/Q mixer but a straight A/D sampling at 122 MHz. The I/Q is generated by the DDC. My TCI/BR 8174 does the same thing at IF.
I wonder if a simple low pass filter would help the fT1000 to limit lower frequency crud from getting into the front end.

[Steve - K4HX]

A/Ds have jitter, which is roughly analogous to phase noise. If the HPSDR designers were smart, the A/D rate was selected so that during down conversion much of the noise is out of band.

If a preamp is not being overloaded, placing a filter in front of it will make no improvement. The FT-1000 has a bunch of front-end bandpass filters that are switched in for each ham band.

Using a preamp with a 1 dB NF is overkill on 20 meters. Unless you are using a very directional antenna, the incoming noise (galactic, atmospheric and man made) is equivalent to a noise figure well above 1 dB.

[KM1H]

Typical rice box noise figure at 30MHz is 12-15dB. My souped up 75A4 is 6dB without a preamp and the performance differential is dramatic.

For the TS-940 where I replaced the IMD prone general purpose band range diodes with PIN's, I switch in a 1.5dB NF @ 11dB gain, high IP3 preamp, which brings the overall NF down to a managable level. An identical one can also be cascaded and has helped on a few occassions where sky noise on 10/12M was exceptionally low.

For 10M AM these days I use a souped up HRO-60 which hears better than the stock TS-940 or 950SD. I had a 1000D for awhile in the early 90's and got rid of it as the receiver performance was much worse than the TS-940.

[WA1GFZ]

The Racal RA6830 increases the noise floor when I connect it to the T12 on 10 meters. It also has about 12 dB noise floor. I would think it could use a few dB of gain on 10. Racal had a step attenuator on the front end piggy backed on the input board of the DF receivers so it would be easy mount a nice high dynamic range push pull Norton anp with a pair of 2N5109s in that spot. 

[K1JJ]

"There is a guy in UK selling high dynamic range tunable preselectors"

Frank,

Do you have a link to this bloke? Is it auto or manual?   I'm getting to the point of wanting to try a preselector since there is really nothing on the front end for selectivity in stock form. With the 40M stack, it's like having a freight train of BC stns rammed down the front end. I'm surprised the Mercury works as well as it does.

I should search the HPSDR archives, but has anyone implemented an available preselector with Mercury that is auto-controlled?  I'll bet some of the UK or German guys have already, but some of their creations can be homebrew nightmares to duplicate.



T

[w3jn]

You need a Harris RF-551A pre/post selector.  These were used in full-duplex HF installations; the transmitter and receiver were spearated by 3 or 4 MHz... the exciter drove a RF-551A which drove a 1.5KW leenyar.  Another RF-551A was used ahead of the RF-590 receiver.

Then there's this little gem which I found when photogoogling for a RF-551A http://www.radiocronache.com/2009/11/rf-preselector-or-not-tiny-scr-digisel-vrf-rf-551a/

[WA1GFZ]

Tom,
Clifton Labs makes a nice BC filter Warren K2ORS has one. Check back through HPSDR for the link to the guy who makes the preselectors. They are not cheap and computer control. Not sure if anyone has interfaced it to HPSDR.
You should see the Flex 5000 at w1aw get trashed by BCI

[WA1GFZ]

My collins unit is like the 551. My cubic is all relay switched in 10 KHz steps.
they usually have a 10% bandwidth with skirts down about 45 dB or better.

[K1JJ]

I did some reading on the  Harris RF-551A and it looks like a winner. It says remote controlled via BCD.  What does this mean? Can it be easily interfaced to the computer/Mercury/PSDR software or will it need Mr. Goldberg's assistance?

How much do they go for and are they ever available? I see none on the web right now.



T

[Steve - K4HX]

Go nuts and get a K&L - 5% BW. These can be controlled by RS-232, GPIB and BCD. BCD is binary coded decimal.