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Old 27th Jul 2017, 11:18 pm   #1
Wendymott
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Default HF Transceiver front end

Hi peeps...... After my 40 M ssb transceiver, which was really a test bed for a more ambitious Multi band HF Version. I am now well into development of the new version.
At present I am short of a little receiving sensitivity, the spec for the FT817 is say 0.5uV on HF bands........... my effort is WAY below that, although with present noise levels on HF It may not be a bad thing.
I attach two photo's.. The first is the 5 band HF Filter, each tuned for a nice flat top "in band" ... note Top band is missing... as I have no interest there.
Reed relays switch each band, and one for In / Out reversal.
The filter is bi directional to enable the TX to be filtered before amplification.
The second picture is the RF front end. At present there is a peak in the 40/20M region.. I suspect because the RF choke is not damped the schematic shows 47uH but it is 100 uH. As far as I remember the gain is approx 16 db.
The use of the fet is for AGC control, but I suspect more learned than me may be able to suggest a better solution.
Maybe someone can suggest a place to look for a better solution.
Thanks in antisipation
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Old 27th Jul 2017, 11:37 pm   #2
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Default Re: HF Transceiver front end

To be honest, I think that some HF receivers are "too sensitive". Those figures at 0.5 microvolt are more suited to VHF rigs. I have made a few HF receivers over the years and have given up well short of the commercial sensitivities and still had acceptable results. Some will argue the case I expect.
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Old 28th Jul 2017, 7:49 am   #3
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Default Re: HF Transceiver front end

Wendy,

I've always thought that sensitivities at HF much below 2uV are a waste of time for SSB mode with around a 2.4kHz bandwidth, given local noise levels for most UK locations - particularly in towns/cities. Note that if you switch to CW with say a 100Hz bandwidth that figure is going to be a lot lower simply because a narrower bandwidth picks up less noise power.

The thinking on HF receivers used to be (still is?) that far more important than raw sensitivity is an ability to handle very high signal levels without intermodulation. That used to be particularly true on the 40m band with lots of strong broadcast stations around. Intermod just introduces far more "noise" into the system, and it can't be filtered out.

So the philosophy adopted was generally ditch the RF amplifier altogether and simply go into the front-end at the mixer stage. And that mixer should be a very high level switching mixer which is as linear as possible. And only after the mixer and the IF filtering do you then start applying gain. That strategy keeps the system as linear as possible - and thus keeps the intermod problem as low as possible.

You can easily test for a receiver limited by intermod by just introducing a linear (i.e. passive RF attenuator between the antenna and the receiver input. A poor receiver will actually get quieter as the attenuation is increased, and the S/N ratio will improve.

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Old 28th Jul 2017, 3:23 pm   #4
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Default Re: HF Transceiver front end

Hi Richard.
The First mixer/ modulator is a MC 1469, as is the second. But i get what you and Alan are saying.
I pulled out the FT 817 manual.......I am going to do some comparison tests between my effort and the 817.
One of my other projects is to make a SSB signal generator, based on the current design, but purely as a signal source, calibrated output. Single tone modulation both LSB and USB.
Maybe I might switch to that and put the "rig" on hold for a few days.
I think the days of proper HF communication are numbered unless you are deep in the black forest or the moon .. with the QRM/QRN around now.
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Old 28th Jul 2017, 7:02 pm   #5
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Default Re: HF Transceiver front end

I second the "RF amplifier not needed" approach: the last 'serious' HF receiver I designed used a switched set of 'Cohn' minimum-loss bandpass filters [basically a chain of alternate top-capacitance and bottom-inductance-coupled tuned-circuits] fed into a packaged double-balanced diode mixer [SBL1/MD108-style] which was driven rather hard by a couple of hundred Milliwatts of LO source [a digital synthesizer designed by a guy from Raytheon who actually knew his stuff]

After the balanced-mixer I used a common-base power transistor [2n5109-style, passing a few tens of milliamps] purely to provide an impedance-match between the 50-Ohm output of the balanced-mixer and the crystal filter.

Gain only really happened at IF.

A couple of things I learned:

1] Even though it's a 'balanced mixer', when you're whacking significant amounts of LO power into it, you'll get a significant LO voltage at the output; put a series-tuned-circuit broadly tuned to the LO frequency from the mixer output to earth, or the LO will get into places it shouldn't.

2] Decouple, Decouple, Decouple! Several tens of millivolts of local-oscillator getting to the input of the AF stages caused some deeply odd results.

3] Put a parallel-tuned trap tuned to the IF in the feed from the antenna to the mixer .

4] A crystal filter isn't always symmetrical despite what Herr KVG says. We tested a batch of them, some worked better 'the wrong way round'. And if hit with strong-enough signals the piezoelectric process itself can become a source of crossmodulation!
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Old 28th Jul 2017, 7:12 pm   #6
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Default Re: HF Transceiver front end

I can offer a few comments on the circuits if that helps. The RF amp circuit does look a bit strange with the capacitive tap at the input and the choice of a dual gate mosfet with BJT follower. The bias control of the BJT looks poor and it is typically only going to be biased at a few mA. It doesn't look like a circuit that will have strong signal handling or a flat frequency response or a low noise figure. Normally, HF preamps are switchable and they are often BJT based. You should be able to get 50R in/out (or similar) with flat gain, low noise figure and decent signal handling. But it's worth making the amp switchable as in many commercial ham HF radios if you feel you want an RF amplifier for some bands.

The only HF radio I have here is my old Trio TS430S and this was never a great radio in terms of overall RF performance. However, it was very sensitive, far too sensitive and I think the noise figure was about 6dB typically. It also had a switchable attenuator (20dB?) and when this was switched in the radio was sometimes a bit too deaf. So it either had too much or too little sensitivity for some scenarios. I kept meaning to put a permanent 5dB attenuator in the Rx path and change the 20dB attenuator to 15dB but never got round to it. I haven't used this radio for years and kind of gave up on ham radio in the 1990s.

Note that I think the TS430S had a signal path of BPF>>FET Mixer>>IF crystal filter>>IF amplifier so it didn't have an RF amplifier. It just had the active FET mixer yet it managed a noise figure of just 6dB on some bands.

From memory, on some bands it could achieve 0.25uV (EMF) for 10dB S+N/N and this was much too sensitive.
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Old 28th Jul 2017, 9:18 pm   #7
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Default Re: HF Transceiver front end

There is an article by John Thorpe (designer of the AR7030) somewhere on the web that explains a lot about signal path design. A quick search found this http://www.ab4oj.com/test/ar7030comments.html interesting too.
 
Old 28th Jul 2017, 10:17 pm   #8
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Default Re: HF Transceiver front end

I have to agree, if you're going to build something, build something adventurous!

The MC1496 is an old stager, but it's an awful long way behind what you could easily build. Look up the "H-mode mixer" by Colin Horrabin. It's a neat circuit with state of the art performance. The amateur radio industry latched onto it several years ago. There's one in the AOR 7030 mentioned above, there's descriptions af vaersions in "tech Topics" by Pat Hawker. Anyone can build them.

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Old 28th Jul 2017, 11:09 pm   #9
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Default Re: HF Transceiver front end

I think the lossless feedback/Norton amplifiers are still one of the best RF amplifier choices if you want a switchable RF amplifier. Have a read of this (now classic?) paper by Dallas Lankford.
http://www.thegleam.com/ke5fx/norton/lankford.pdf


The noise figure, frequency response and signal handling performance of his push pull version is extremely good. I think several companies now make these amplifiers as kits and they look just like the Lankford prototype in terms of schematic and PCB layout with the central screen.

It can be fiddly getting the transformer turns/phasing correct for these amplifiers but they do work well. Back in the early 1990s at work we used the original Norton design using BFT66 transistors and this achieved very good performance. One downside is that these amplifiers have poor reverse isolation and the 50R match is heavily dependent on the circuit the amp interfaces to. I think the first edition of the Digital PLL Frequency Synthesisers book by Rohde has a section on this amplifier and this is probably where we first spotted it at work.

I used to have a bag with about 50 BFT66 devices in it but I misplaced it many years ago. They would be worth a lot of money these days
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Old 29th Jul 2017, 6:51 am   #10
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Default Re: HF Transceiver front end

There was a full HF receiver front-end and IF design done by Jacob Makhinson in QST. I think it used push-pull Norton amplifiers, but he used the Racal style ring-of MOSFETS mixer (originated by Rafuse) It would have been better with the H-mode type which he'd probably not heard of. I was doing some work for the ARRL back then and used to get advance copies of all sorts of stuff.

The CDG2000 is worth a look as well.

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Old 29th Jul 2017, 7:53 am   #11
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Default Re: HF Transceiver front end

I guess if a common Norton amplifier is used after a suite of narrow preselectors for each ham band then the IP2 performance of the push pull version wouldn't be needed. So the design could be the single ended version. Much easier to make.

I'm not sure what the frequency plan is for the radio but the preselector (or another filter) will have to provide image (and IF?) rejection. How much image rejection does the current preselector provide on each band? Also, it seems odd that some of the coupling caps are shorted in the schematic. Are these trap/notch sections?
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Old 29th Jul 2017, 10:55 am   #12
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Default Re: HF Transceiver front end

It may be of interest that Spectrum Communications offer a multi-band front end filter kit. Not as much fun as designing your own I admit. I made one up a while ago for my (very) long project for a multi-band HF Rx that I started many years ago. One day I may get round to finishing it. The multi-mode IF strip etc is based on a design published in Radcom sometime in the eighties.
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Old 29th Jul 2017, 11:37 am   #13
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Default Re: HF Transceiver front end

Most receivers seem to have more gain than you can possibly make use of, the best I have seen is the Pye Westminster which has a preset gain control on the 2nd IF input, you can set it so that any signal strong enough to be heard will appear even with the squelch right open set to a just audible position with the gain control.

Peter
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Old 29th Jul 2017, 6:52 pm   #14
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Default Re: HF Transceiver front end

Hi peeps...... Thanks for your very informative replies....First Tanuki.. I bought 2 modules with KVG filters x3..as per a previous posting, this also contained the MC1469 Modulators.
David. I decided to "recycle" the MC1496's as they have given me a good understanding of DBM's and they stay stable.... I tried other mixers including the "H" mode, but I found I could not keep them balanced on transmit, thus I kept getting LO leakage..
Jeremy... The shorts across some of the filters on the schematic was to remind me to add links if necessary on the pcb. On the 15 and 10M bands the links are required.
After I had posted the original opening.... it hit me that the Input "shunt" cap values were all wrong..especially the 10m one.. 470pf @ 29 Mhz = approx 11 ohms reactance. Thus the reason for serious loss of input at 10m.
I will go away and get out my filter test jig and re evaluate the values.
I do understand that the Gain should be in the I.F... and that the front fet should be replaced maybe with a pin diode atten..or AGC in the If amp..
Please note.. I am not trying to "invent or reinvent the wheel" but as an exercise.. in keeping me entertained post retirement. Going by the received reports of my 40m transceiver, I was very pleased and decided to expand its coverage.
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Old 29th Jul 2017, 9:16 pm   #15
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Default Re: HF Transceiver front end

Just had the reps in for a visit last week. KVG was bought out by the Dover group many years ago, then sold to Vectron and what I hadn't realised was that they'd bought themselves out and are now a privately held family business again. So they're on the go again and out looking for business.

Long reckoned to be the best of all the crystal filter firms, though I think CEPE had the edge on them, but CEPE (Part of Thomson-CSF) weren't known in the general market.

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Old 30th Jul 2017, 2:56 pm   #16
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Default Re: HF Transceiver front end

Quote:
After I had posted the original opening.... it hit me that the Input "shunt" cap values were all wrong..especially the 10m one.. 470pf @ 29 Mhz = approx 11 ohms reactance. Thus the reason for serious loss of input at 10m.
In theory having a 470pF shunt cap is fine for a (50R) capacitive tapped bandpass filter at 28MHz. However, it would mean the bandwidth would be relatively narrow for a typical design. Typically too narrow to cover the whole of the 10m band? Also, the unloaded Q of the components in the filter would become a bit more more critical wrt insertion loss. Also, a typical 470pF leaded ceramic cap won't behave as a 470pF cap any more at 28MHz due to the lead and package inductance. So it could typically behave as a 550pF cap at 28MHz, which would mean even less bandwidth in the filter when it was built and tested
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Old 30th Jul 2017, 3:02 pm   #17
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Default Re: HF Transceiver front end

I checked in my archives and I still have all my old Eagleware (RF CAD) simulations of Norton RF amplifiers if you wanted so see how they perform? eg noise figure, port VSWR, gain, reverse isolation, stability analysis and distortion performance.

Also, each turns ratio option is tested, theory against simulation for gain etc.
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Old 31st Jul 2017, 12:01 pm   #18
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Default Re: HF Transceiver front end

Hi Jeremy. Thanks for the offer...and I am not being dismissive.... but it is a little further than I wanted to go...I holds my hands up.. and say ... it frightens the "heck" out of me.... however I revisited the filters yesterday...... With a 150pf cap in that position both input and out at 20/15 and 10..... I was getting a much better response . I can post photo's of the responses later. I did the testing on a Single filter jig, so I have to change the values in the filter block before I can be 100 % sure.
This may be right or wrong, but I shunted the input with a 50R resistor to see what the real impedance was and it was quite near 50R... as a 3db reduction of signal was noted.
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Old 31st Jul 2017, 6:11 pm   #19
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Default Re: HF Transceiver front end

OK but just to clarify my earlier post...

Having a 470pF cap (with -11R reactance) as the first shunt component in a capacitive tapped BPF at 28MHz doesn't automatically mean that the insertion loss is going to be seriously high. As long as the rest of the filter is (re)designed around these 470pF caps then it should be possible to make a capacitive tapped BPF for the 10m band like this. The captap network is very versatile.

Having simulated a fresh filter designed for 470pF caps, I'd expect the insertion loss to be about 2dB allowing for typical component Q in a dual resonator filter similar to yours. The downside with using 470pF caps will be that the component parasitic inductance will cause issues with the stopband up at UHF and it will be harder to find components with adequate unloaded Q to preserve the insertion loss. But even with basic/cheap parts I'd expect the insertion loss to be in the ballpark of 2dB. Not great, but not what I would class as seriously high insertion loss.
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Old 31st Jul 2017, 6:28 pm   #20
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Default Re: HF Transceiver front end

As I mentioned earlier, the other curious thing is the capacitive tap at the input to your RF amplifier. Normally, a dual gate mosfet is capable of achieving a low noise figure. But with a capacitive tap at the input you are throwing this all away. If I assume your preselector filters are designed for 50R and they connect to this amplifier then the match will already be poor but also the voltage going into the amplifier will be tapped down in the amplifier with that extra 47p and 220p. This ratio will knock the voltage down by factor of 5 or so and so the noise figure of the overall stage will be degraded by a ballpark 14dB. You can't claw this back by adding gain afterwards. The dual gate mosfet also effectively has a tuned circuit in the drain caused by the inductor and the capacitance of the mosfet and the BJF after it. So it will give a peaky response somewhere in the HF band. The signal handling will be poor too. I would choose something else here, even if you don't want to use a Norton amplifier. There are loads of designs out there that provide a reasonable noise figure, good 50R match and decent signal handling
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