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Author Topic: Caascode Triode - Triode Receiver RF Amp  (Read 5749 times)
KB1GMX
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Posts: 698




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« Reply #15 on: January 29, 2013, 05:30:53 PM »

Its the right link but the editor (webbased) translated some trialing cruft to noise.

http://www.hammarlund.org/files/NUVISTAPLUG.pdf

just use that.

Allison
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G8HQP
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Posts: 119




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« Reply #16 on: January 30, 2013, 06:06:22 AM »

Quote from: G3RZP
'What is the dominant noise source?' It may well not be the enr of the tube itself, but the Rd of the tuned circuit.
Yes, unless the front end was designed for lowest noise rather than reasonable selectivity and reasonable antenna match (which is more usual for HF) the dominant noise source will be the dynamic impedance of the grid tuned circuit. Grid noise can start to be a problem at the upper end of the HF range. It is only in really old (octal and earlier) valve receivers that valve shot and partition noise may dominate, and some of that may be from the mixer.

Simply putting in a higher gain valve (or cascode) is likely to cause AGC problems, as the RF valve will be nearly cutoff long before the IF valves (often 6BA6) have reduced gain by much. You may need to attach the RF grid bias to a tap on the AGC rail.

Another issue is tightness of antenna coupling. Most general coverage receivers have to use fairly loose coupling in order to get reasonable tracking and not allow the antenna to detune the grid circuit. Tighter coupling can reduce noise (by loading the tune circuit dynamic impedance down) but then the antenna has to be carefully tuned.
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G3RZP
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« Reply #17 on: January 30, 2013, 07:56:22 AM »

>Simply putting in a higher gain valve (or cascode) is likely to cause AGC problems, as the RF valve will be nearly cutoff long before the IF valves (often 6BA6) have reduced gain by much. You may need to attach the RF grid bias to a tap on the AGC rail.,<


In the R4C, the AGC is fairly complicated, as it uses a transistor detector and DC amplifier for the AGC on the IF and the 2nd mixer, AGC being applied to the oscillator grid (g1) of a 6BE6, rather than to g3 as is more usual. However, the RF stage is on a separate AGC line. AGC is applied to the RF amplifier, second mixer and 50kHz 6BA6 IF amplifier, and the RF AGC is delayed. AGC threshold is typically 1 microvolt, and the output is specified to increase by 3dB max for input from threshold to 100 dB above that.

>Another issue is tightness of antenna coupling. Most general coverage receivers have to use fairly loose coupling in order to get reasonable tracking and not allow the antenna to detune the grid circuit. Tighter coupling can reduce noise (by loading the tune circuit dynamic impedance down) but then the antenna has to be carefully tuned.Another issue is tightness of antenna coupling. Most general coverage receivers have to use fairly loose coupling in order to get reasonable tracking and not allow the antenna to detune the grid circuit. Tighter coupling can reduce noise (by loading the tune circuit dynamic impedance down) but then the antenna has to be carefully tuned.<

The R4C uses one permeability tuned coil to cover the 1.5 to 30 MHz range in the RF amplifier grid, and another in the RF amplifier anode circuit, so the coupling is likely to be fairly loose, and a far from optimum L/C ratio over much of the range.  For instance, it uses around 600pF on 80 metres. There's no tracking involved though - the circuits are just ganged and with their own tuning control.The sensitivity is specified only from 3.5 to 30 MHz. It has two tuned circuits, and achieves 70dB of image rejection below 23 MHz and 60dB above 23 MHz. The first IF  is 5645kHz.


Chuck, there are standard equations for the equivalent noise resistance of tubes as amplifiers and mixers. I was able to take a stab at the currents from the tube data sheets and the voltages given in the R4C manual - I've never had an R4C, but someone sent me a copy of the manual when he had a question - which was back in 1983 - and I never throw manuals away!

The biggest unknown is the conversion transconductance of the mixer, but I'm guessing it will be about one quarter of the transconductance as an amplifier. The RF amplifier is an easy one to analyse, and the screen volts on it will be a little low for the 6BZ6, which is the first type I'd try. The advantage of just being able to swap a tube is that it's easy

Noise figure is derived from the handbook sensitivity figures. Thermal noise (KTB, which is Boltzmann's constant times temperature in Kelvins times bandwidth in Hertz) is -174dBm/Hz at room temperature. The noise bandwidth is about 2.5kHz, which is 34 dB great than 1 Hz, so the noise floor for a 0dB noise figure receiver is -140 dBm. It gives a 10dB (S+N)/N ratio for 0.25 microvolt, which is -119dBm (being American, it will use actual antenna volts rather than European emf antenna volts), so the receiver noise floor is 10dB below that or -129 dBm. That's 11dB worse than the theoretical, so the noise figure is 11dB or better.

You haven't said which band or bands you need the extra sensitivity on. Although it's worth plugging a 6BZ6 in, I'm inclined to think that an external preselector may be a better route to go down, especially if it's only one band. One has to be careful though, because increased front end gain often causes other problems such as intermodulation, and this where reducing RF gain can help. Certainly here, measurements over some years have shown that a dynamic range of 95 to 100dB is all that is needed: what matters is being able to change where that 95dB starts, and the easiest way to do that is an antenna attenuator. Another advantage of the external preselector would be the extra tuned circuits whioch will reduce the level of adjacent band signals.

I hope this helps. Let me know if you have any more questions.

73

Peter G3RZP
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AA5WG
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Posts: 493




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« Reply #18 on: January 31, 2013, 11:11:37 AM »

Alison and Peter,

Thank you for the new link regarding the Nuvistaplug.  I remember years ago seeing this paper.

I will read the material.

Peter,  thank you for the information you provided.  The bands in use were 40, 30, 20 and 15.  I like the R4C but it has a ton internal generated noise.  I will read your comments more than once and add them to my to do list.

Again, thank you.

Chuck
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G3RZP
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« Reply #19 on: February 01, 2013, 02:48:43 AM »

Chuck,

One reason for it appearing noisy could well be the very flat AGC response , Once you are 23dB above the noise floor, there's only a 3dB rise for 100 dB more signal. Now especially on the lower bands, you have to have a very inefficient antenna to get that low. In a quiet rural location, a 40m dipole will give around one microvolt of noise as a mixture of man made and galactic, depending on time of day. Now that means that the R4C will be into AGC action pretty well all the time and you can then expect only a 3dB change in noise for large signals. This is a problem with excessively flat AGC responses.

You might well find that switching AGC off and backing off the RF gain control makes things appear better.
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AA5WG
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« Reply #20 on: February 04, 2013, 06:10:24 PM »

Peter,

Again, thank you for the information.  I will study this and determined a route to take in reducing some noise.

Chuck
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