TheRadioBoard

Vladn, a while back you wrote:


Any news on this? I'm itching to build another shortwave receiver.

I was also thinking with its AGC action this design might be usable as the back end of a superhet. Just stick a low-distortion mixer in front of it, let the AGC regulate the RF gain (actually IF gain) down to an appropriate level for the regenerative detector, and you have a superhet. The frequency-compensated regen level would make a regenerodyne (fixed LO and tunable IF) approach feasible without adjusting the regeneration control, though I might want a fixed IF to experiment with crystal filters.

I am away from home till early August (visiting Russia actually ). Some time ago I started working on a high end solid state regen. It works OK but not quite as good as my 6AS6 tube RX, so it is not quite ready yet.

Regenerodyne is certainly a good fit for the frequency compensated regenerative detector. Some time ago I posted a balanced autodyne mixer concept in the tube RX section of the forum, I want to try it out some day and build either a regenerodyne (variable IF) or a plain superhet (fixed IF) with a regen detector using either tubes of transistors.

I will be interested to see what circuit you come up with for a transistor regenodyne! My first instinct would be to substitute FETs for the tubes and go from there.

_________________
Norman,
Over 30 years
as KA1GUK

If you want to build a solid-state Regenereodyne with a fixed regenerative IF stage, here you are!

Schematic:

http://www.uploadarchief.net/files/download/shortwave%20supergainer.gif

Couple of questions on your design, DrM.

I've heard super-gainers are quite prone to overload, more so than normal regens, because of the RF gain and mixer gain ahead of the detector making it really easy to present the detector with too strong of a signal. Is that your experience?

Also I notice you're using a BJT as the front-end mixer. Do you notice poor dynamic range or IMD artifacts when strong signals are present? I've often thought a cascode JFET mixer (or dual-gate MOSFET) would probably be a reasonable front-end mixer for this kind of receiver.

And to vladn: I continue to look forward to your high-performance, solid-state regen design. Having built a couple of Kitchin regens (and several other designs), I think your design (with frequency-compensated regeneration level and AGC) seems like a big step up in performance from more traditional regen designs.

@qrp-gaijin:



Yes, it is also my experience that the regenerative IF stage is easily overloaded. With adjusting RF Gain potmeter P1 or by moving the IF tank circuit L2/C5 away from the ferroceptor of the regenerative IF stage, I can avoid overloading. Overloeading makes receiving SSB signals difficult while the regenerative IF stage is oscillating.

I also notice some IMD when strong signals are present. It happens with strong signals in the passband of the antenna tank circuit. You hear a carpet of noises. When it happens, I adjust P1 so that the RF Gain is lowered.

A supergainer, a superhet with a regenerative IF stage, is not for nothing called a supergainer. Because the mixer has some gain, combined with the high gain of the regenerative IF stage, the gain is tremendous. The RF stage of my regenerodyne is a source follower for a good impedance match between the aerial tank circuit and the low impedance RF input at the base of the bipolar mixer.

One thing that worked better than expected in my tube regen is the AF-derived AGC of the RF buffer. The tube used in the RF buffer 6HA5 has a semi-remote characteristic and provides substantial gain range without objectionable IMD. It works so well that I rarely have to operate the RF attenuator in any mode (SSB/CW/AM).

However so far I have not been able to get a comparable gain range from an equivalent single transistor buffer. Because the RF buffer is located before the single selectivity element in a regen the IMD effects are quite noticeable. Reducing the RF buffer current in the transistor version attenuates the strong station but also adds parasitic reception channels. Some work is needed to bring the RF AGC performance closer to the tube version with no major increase in the complexity of the RF stage.

Might you elaborate on what a parasitic reception channel is, and by what mechanism reduction of the RF buffer current intoduces such parasitic reception channels?

@QRP-gaijin:


Reducing the current of the RF input buffer stage biases it into a more non-linear region. Andt that is the cause of IMD.

He means off-channel signals that, because the front-end selectivity is wide, appear at the mixer input. It's called intermodulation distortion when the undesired signal(s) mix(es) with the desired signals or one another to produce mush. Reducing current flow through the buffer reduces its power-handling capability, making it more susceptible to overload; in effect, becoming a mixer itself when strong signals drive it into non-linearity.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I've been thinking about using a PIN diode or perhaps a photoresistor as an electronically controlled RF attenuator with a better dynamic range for the AGC in a transistor regen.

Also in a tube version I want to try using a second 6AS6 in the RF stage and use g3 for AGC which should provide even better RF dynamic range.

Just a periodic check on this topic and a continued expression of interest. Any news vladn?

I'm still thinking about using such a design as the tunable IF in a superhet.

Switching to this thread (since the subject is not Clapp specific). This is a continuation of this thread:
http://theradioboard.com/rb/viewtopic.php?t=4472
and the original thread on tube branch related to hybrid feedback regens:
http://theradioboard.com/rb/viewtopic.php?t=3714

I have put up two circuits illustrating a minimalistic jfet regen using the hybrid feedback. The second one adds a Vackar divider and is more suitable for SW use. Please note - I have not verified these as drawn although they are relatively close to what I used for testing (also see comments below on the gate voltage regen control):





I start at 1:3 ratio and adjust experimentally. The main purpose of the Vackar divider is balancing disturbances introduced by the jfet gate and drain. The secondary purpose is an additional mean of the loop gain control.


There are three subtle differences:
- oscillation amplitude is better controlled with positive bias at the gate/grid (I mentioned that before somewhere in the tube section);
- the RC time constant shaping AF bandwidth is fixed;
- (possible) simulation shows that there is some compensatory effect of simultaneously increasing Ids and decreasing Vgd reducing frequency shift, not sure it is simulation artifact or real effect, testing is needed.

jfet has a relatively high output impedance that drives a relatively low impedance feedback network, hence the signal amplitude at the drain is small. If you increase the load impedance by inserting a series element in the drain circuit the amplitude at the drain will increase.

Look, we went over this several times in other threads . Let me summarize:
- I have tested the method on two very distinct circuits and got over a magnitude improvement compared to conventional feedback by removing the linear term alone. I documented this to the best of my abilities - check the graphs on pg23 and pg25 of my main report, these are actual measurements;
- the remaining higher order terms are hard to predict because they depend on complex physical phenomena in the resonant circuit that determine it's Q(f);
- no I do not know how to remove the remaining terms;
- I do not find them objectionable in practical use, take a look at videos that I made;
- there is always room for improvement in any design...

Thank you! I look forward to trying out these circuits in the near future.


I'm not complaining. Earlier I may have been overly insistent on the possibility of removing the nonlinear terms; now, I'm simply interested about their origin. In particular, the K3NHI data (on the regenrx list) indicates what appears to be more nonlinear behavior than you measured. Looking at K3NHI's prototype circuit layout there seem to be possibilities for stray capacitances and inductances which may account for some of the additional nonlinearity.

I will report my experiences when I get around to building one. Probably, I will use a varactor in parallel with C2 for in-place tilt adjustment.