EMRFD Message Archive 15027
Message Date From Subject 15027 2018-08-27 09:42:29 g4jxx Hycas IF amp
Is it possible to add a crystal or bpf before the agc amplifier to help stop hash from affecting the agc.
15028 2018-08-27 11:29:42 kb1gmx Re: Hycas IF amp The answer is yes. Its called a tail end filter.It helps with wide band amps having device noise in band.Generally the hycas is quiet though its gain is very high.Why are you getting hash there? Is it at the input and amplified?Allison 15030 2018-08-27 12:49:18 Mike Hadley Re: Hycas IF amp
Without any input the hash peaks at 5.6MHz tailing off after 10.0MHz. It can be seen after the first stage then amplified. In my case the level of hash does not cause any agc action. I have followed my own Hycas board with a four pole 3.6KHz filter which rids it completely. I posed the question as it had been raised with me by others.
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The answer is yes. Its called a tail end filter.It helps with wide band amps having device noise in band.Generally the hycas is quiet though its gain is very high.Why are you getting hash there? Is it at the input and amplified?Allison
15031 2018-08-29 11:02:27 kb1gmx Re: Hycas IF amp Most common issue is incidental feedback causing regeneration or outright oscillation.The amp wide open has 120DB of gain, at that level of gain it takes very little tomake it noisy or oscillate. Its also enough to see the noise generated by theinput devices.The intent I believe was not running it wide open but throttled and still have veryhigh agc range.However I can see why it would need a output bandpass. Mine I run it with the agcactive so peak gain is lower and so its the noise. that and shielded container.Allison 15032 2018-08-30 20:27:21 rcbuckiii Re: Hycas IF amp As Allison said, the amp needs to be in a shielded container.I have built 2 of them, the kit from Roger and a board I designed with all SMT parts that is quite a bit smaller than Roger's board.Testing both of them in a receiver I built revealed they had to be in a shielded housing to prevent noise pickup. I have a switch on the front panel to turn the AGC on or off. When the AGC is on, the noise level is quite a lot less due to the reduced gain. I have the gain control on mine set about 3/4 open.Ray,AB7HE 15033 2018-08-31 00:26:06 mhadley157@gmail.... Re: Hycas IF amp I can see the importance of shielding. My enclosure is made of zinc coated steel with smb connectors and feed throughs for power etc. The newer board (from me) shows a noise peak around 6 meg. Turning down the gain helps but I find that a simple filter at the output works wonders. This now is just a 2 pole 4KHz crystal filter matched to 50 ohms. Maybe the next board revision will include this.
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15034 2018-09-01 15:19:02 kb1gmx Re: Hycas IF amp A noise peak also suggest the chokes or inductors used are self resonant at that frequency.That may means many things but not helpful unless your filter is also around that frequency.The reason is noise mixes with signal and you get signal plus noise and even a post filterallows that signal plus combined noise through degrading the noise figure.Like I said who or what application needs 120DB of IF gain?Allison 15035 2018-09-02 10:23:44 Mike Hadley Re: Hycas IF amp
Using the IF gain control lowers the gain (plus noise) to acceptable levels. Changing the chokes made no significant difference. There have been well over two hundred of these boards built and put to use. Perhaps the noise issue is of little consequence.
Regarding the 120db gain (and noise) issue perhaps Wes would be the best person to comment.
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A noise peak also suggest the chokes or inductors used are self resonant at that frequency.That may means many things but not helpful unless your filter is also around that frequency.The reason is noise mixes with signal and you get signal plus noise and even a post filterallows that signal plus combined noise through degrading the noise figure.Like I said who or what application needs 120DB of IF gain?Allison
15036 2018-09-05 04:32:10 w7zoi Re: Hycas IF amp Hi Mike, and gang,I'm a little confused about this 120 dB number. I never measured anything even approaching this. I looked at the info I have posted on my web site. If you go to the "technical notes" in the w7zoi.net site, you quickly come to the section on the hybrid cascode. Click on that and go to the paragraph below the first photo. Within the text is another link that discusses stability problems. Near the end of that section is a gain number of 58 dB. That may be a few dB below the original gain, so that measurement was done after the gain had been reduced with some resistor changes, but those were changes of a few dB and not 60 dB. There was never a gain of 120 dB.Although gain was never all that high, the gain reduction available was around 100 dB.While the gain is modest, there can be a lot of output noise if the IF output is observed in a moderately wide bandwidth. Once it is applied to a detector with a baseband (audio) output, the noise power is within reason, usually a result of low pass filtering within the audio amplifier.There are many different kinds of gain, and many ways to measure them. When I evaluate an RF circuit, I usually measure transducer gain, a.k.a. insertion gain. If using an oscilloscope to measure an IF amp, the 'scope is fitted with a 50 Ohm termination at the input. A 9 MHz signal generator is attached to the terminated 'scope with a coax cable and the generator output is adjusted for a signal of around -30 dBm. That's a low signal of on 20 mV pk-pk, small but still measurable with a 'scope. It's easier with a spectrum analyzer.Having established a level at the 'scope, the coax from the generator is disconnected and the IF amplifier is inserted between the generator and the terminated scope. Power is applied. AGC is turned off in the IF amp, but manual gain is set to max. We now, of course, see that the amplifier is over driven. But we now start clicking in attenuation in the signal generator output. When we get to 58 dB of attenuation, we have the same signal level that we originally saw, 20 mV pk-pk. The transducer, or insertion gain is, hence, 58 dB.Note that this is an "insertion" measurement in a 50 Ohm environment rather than the insitu probing measurements we often do with an oscilloscope. This may well be a defining difference between RF measurements and those in a high speed analog world.The power, P, in the 50 Ohm termination in milliwatts is 2.5(V^2) where V is the peak to peak voltage across the 50 Ohms. The level in dBm is just 10Log(P). Don't use 20 here, for the voltage has already been squared!The HyCas QST paper that Jeff (wa7mlh) and I wrote described a complete IF amplifier. With the help of Roger (ka7exm) we made boards available, and even full kits. That was nearly a requirement if one was to publish in QST at that time. The primary goal that Jeff and I had was not a description of a complete "project." Rather, it was to present the combinati 15047 2018-09-06 00:00:04 Kevin Murphy Re: Hycas IF amp If one is measuring the NOISE with a spectrum analyzer and the analyser bandwidth is different from the receiver bandwidth, then a correction factor would probably apply.Say the RX bandwidth is specified at 3 kHz and the analyser is looking at a 3MHz bandwidth, then ~ 60 dB should apply.The AGC detector and demodulator circuits will see the IF noise over their bandwidth, which could well be in the MHz range, hence it is important to limit the bandwidth at the end of the IF.If I remember correctly the early Plessey IF ICs had bandwidth around the 120 MHz, so were incredibly noisy.I think IF noise power can degrade Rx sensitivity, at low front end gains, unless filtering is done in the IF domain, or at least some good filtering in the AF domain..Just my 10c worth... (2c aren't legal tender in New Zealand)Kevin