JFET buffer amplifier for probing IF stages - UK Vintage Radio Repair and Restoration Discussion Forum

6SN7WGTB · 9th Jan 2023, 7:29 pm

Am considering aligning a SONY ICF-SW7600 and it specifies a small JFET buffer circuit to make some readings (see below).

Am I correct that this circuit aims to do two things:
1. Provide as low a loading on the circuit as possible
2. Amplify the small signal to be readable by the frequency counter?

I've also attached the point at which the measurement is to be made - it's on the Source of the 2nd mixer - marked '0' with (1.33) by it.

The 1st IF stage appears to be around 55.38MHz depending upon band.

What I'm puzzled about is that on trying to model this circuit in LTSpice I get more like a 10x attenuation of input. This may be down to my choice of jFET in LTS as I cannot find a model for the 2SK193-E - can anyone suggest one?

Essentially I would like to be sure that (a) my understanding is correct and (b) that here isn't some horrible typo in the schematic, before attempting to use it.

Jon_G4MDC · 9th Jan 2023, 7:52 pm

I think check your FET model.

There is no source resistor so you are looking for a depletion mode FET that will run at an IdSS with Vgs=0. You probably picked an enhancement type?

My knowledge ends (or it might be over extended) here. I hope it turns out to be of some assistance. I am sure you are right that the buffer is just to provide minimal loading and a bit of gain to make a deaf counter read on a low power oscillator.

buggies · 9th Jan 2023, 8:17 pm

Similar thread with some info which might help here:-
https://www.vintage-radio.net/forum/...d.php?t=129433

Bazz4CQJ · 9th Jan 2023, 8:33 pm

That type of buffer amp has been discussed quite a few times on the forum. It's a circuit which has been published many times (with FET's of different kinds). However, a number of people, especially those with a mastery of LT Spice, say that such circuits suffer from negative resistance of the FET and are likely to give poor results.

I cannot offer any links to previous threads, but a few of them do exist.

B

6SN7WGTB · 9th Jan 2023, 9:02 pm

My apologies for not searching more carefully before posting.

I'll read the linked thread and revert if I'm still stuck.

6SN7WGTB · 9th Jan 2023, 9:18 pm

So, I am completely incorrect in my assumption that the circuit amplifies the signal for the frequency counter!

In fact it attenuates it by many (20 in the case of the linked circuit) dB? Which is what my LTSpice sim shows...

How does a typical frequency counter work with such a low signal level?

Jon_G4MDC · 9th Jan 2023, 9:31 pm

I think if you read again the consensus was that a Dual Gate MOSFET based probe was a better bet. My own first idea was I would use a BF981 or some such for that, except they have become some sort of gold dust.

Spurious oscillations are a risk all the same. You are not wrong that it is supposed to amplify the LO for the frequency counter.

6SN7WGTB · 9th Jan 2023, 9:35 pm

Quote:

Originally Posted by Jon_G4MDC

I think if you read again the consensus was that a Dual Gate MOSFET based probe was a better bet. My own first idea was I would use a BF981 or some such for that, except they have become some sort of gold dust.

Spurious oscillations are a risk all the same. You are not wrong that it is supposed to amplify the LO for the frequency counter.

Understood, but going through to the linked post and indeed the dual gate MOSFET design also purports to have 20dB of insertion loss?

Bazz4CQJ · 9th Jan 2023, 9:47 pm

The guy who spent quite a lot of time on this was our sadly departed colleague 'Skywave' - Al. About 4 or 5 years ago he ran two threads at different times, one object was to make a sensitive, high-impedance RF millivoltmeter.

He certainly did manage to make an amplifier that had a flat response to something like 150MHz. There was a FET at the front and two BJT's.
I think that this the thread, post 22 http://www.vintage-radio.net/forum/s...977#post896977.

Best to Al !

B

9th Jan 2023, 7:29 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	JFET buffer amplifier for probing IF stages Am considering aligning a SONY ICF-SW7600 and it specifies a small JFET buffer circuit to make some readings (see below). Am I correct that this circuit aims to do two things: 1. Provide as low a loading on the circuit as possible 2. Amplify the small signal to be readable by the frequency counter? I've also attached the point at which the measurement is to be made - it's on the Source of the 2nd mixer - marked '0' with (1.33) by it. The 1st IF stage appears to be around 55.38MHz depending upon band. What I'm puzzled about is that on trying to model this circuit in LTSpice I get more like a 10x attenuation of input. This may be down to my choice of jFET in LTS as I cannot find a model for the 2SK193-E - can anyone suggest one? Essentially I would like to be sure that (a) my understanding is correct and (b) that here isn't some horrible typo in the schematic, before attempting to use it. Attached Thumbnails __________________ Richard \| BVWS & RSGB member \|

9th Jan 2023, 7:52 pm	#2
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: JFET buffer amplifier for probing IF stages I think check your FET model. There is no source resistor so you are looking for a depletion mode FET that will run at an IdSS with Vgs=0. You probably picked an enhancement type? My knowledge ends (or it might be over extended) here. I hope it turns out to be of some assistance. I am sure you are right that the buffer is just to provide minimal loading and a bit of gain to make a deaf counter read on a low power oscillator.

9th Jan 2023, 8:17 pm	#3
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 762	Re: JFET buffer amplifier for probing IF stages Similar thread with some info which might help here:- https://www.vintage-radio.net/forum/...d.php?t=129433 __________________ George

9th Jan 2023, 8:33 pm	#4
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Re: JFET buffer amplifier for probing IF stages That type of buffer amp has been discussed quite a few times on the forum. It's a circuit which has been published many times (with FET's of different kinds). However, a number of people, especially those with a mastery of LT Spice, say that such circuits suffer from negative resistance of the FET and are likely to give poor results. I cannot offer any links to previous threads, but a few of them do exist. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

9th Jan 2023, 9:02 pm	#5
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: JFET buffer amplifier for probing IF stages My apologies for not searching more carefully before posting. I'll read the linked thread and revert if I'm still stuck. __________________ Richard \| BVWS & RSGB member \|

9th Jan 2023, 9:18 pm	#6
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: JFET buffer amplifier for probing IF stages So, I am completely incorrect in my assumption that the circuit amplifies the signal for the frequency counter! In fact it attenuates it by many (20 in the case of the linked circuit) dB? Which is what my LTSpice sim shows... How does a typical frequency counter work with such a low signal level? __________________ Richard \| BVWS & RSGB member \|

9th Jan 2023, 9:31 pm	#7
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: JFET buffer amplifier for probing IF stages I think if you read again the consensus was that a Dual Gate MOSFET based probe was a better bet. My own first idea was I would use a BF981 or some such for that, except they have become some sort of gold dust. Spurious oscillations are a risk all the same. You are not wrong that it is supposed to amplify the LO for the frequency counter.

9th Jan 2023, 9:47 pm	#9
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Re: JFET buffer amplifier for probing IF stages The guy who spent quite a lot of time on this was our sadly departed colleague 'Skywave' - Al. About 4 or 5 years ago he ran two threads at different times, one object was to make a sensitive, high-impedance RF millivoltmeter. He certainly did manage to make an amplifier that had a flat response to something like 150MHz. There was a FET at the front and two BJT's. I think that this the thread, post 22 http://www.vintage-radio.net/forum/s...977#post896977. Best to Al ! B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.