Substituting and matching the JFET's in this VCO circuit - Page 1
Forum section: electronics / rf-microwave

author: djsb
author_rank: Super Contributor
timestamp: October 03, 2021, 12:08:20 pm
content: Hi,I would like some advice on which JFET's to substitute for the U1994E JFET's and how to match them (Schematic attached mentions matched pair delta Vd less than 200mV at 1mA-What does this mean?) in the schematic attached. I also need to work out the coil inductances for the VCO. I'm going to try using LTSPICE and Microcap 12 to simulate the circuit as well.Can a peak DCA75 be used for matching the JFET's? Any help appreciated. Thanks.David.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2021, 01:17:30 pm
content: Matching to 200 mV is not a very stringent requirement. So no very accurate method needed. I would use a simple circuit with gate to ground, drain to some 5-10 V about simillar to later use and a resistor from source to ground, chosen so the about 1 mA flows. Start with a larger resistor and decrase to get some 0.8 to 1.2 mA to do the matching. With a fixed resistor the voltage for that resistor is measured for a few FETs (chances are one would need something like 5 pieces from the same batch to find 2 that are relatively close (e.g. < 200 mV) .A suggested replacement for the U1994 fet is the PN4416 / 2n4416. For which frequency range is this circuit ?Edit:THT JFETs are a bit hard to get. So its more like looking for some RF suitable to92 case fet and if needed adjust the divider for the gate voltage.

author: djsb
author_rank: Super Contributor
timestamp: October 03, 2021, 01:34:34 pm
content: Thanks for your reply. The maximum frequency is 130MHz, so anything around that OK (as close as possible for stability). I'd like to use THT JFET's for prototyping (Would J310's work OK-I have loads of them?). I would probably choose an SMT JFET if making a final product. Just playing around at the moment.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2021, 02:58:36 pm
content: THe J310 has a bit more input capacitance than the 4416, but it may still work.

author: StuartA
author_rank: Regular Contributor
timestamp: October 06, 2021, 11:22:04 pm
content: I was curious about what such an oscillator had been used for, and see that the RE101 sig gen was used for servicing broadcast band AM and FM receivers. The frequency stability claimed for the 101 would be OK for that purpose, but might be wanting for many other applications. Wonder what you are using it for?

author: djsb
author_rank: Super Contributor
timestamp: October 07, 2021, 05:54:31 am
content: I don't own an RE101 (VERY difficult to get hold off) but I'd like to build a LOW distortion RF oscillator and I thought I'd look into this one. If there is anything better, I'd be willing to look at that also.I'd like one of these, so I can test/align various FM tuners I have lying around.

author: StuartA
author_rank: Regular Contributor
timestamp: October 07, 2021, 03:59:05 pm
content: I've been involved with oscillators for various requirements over the years, but nothing remotely connected to servicing FM tuners. Could you explain why this task benefits from a low distortion waveform? My impression (perhaps erroneous) regarding FM tuner set up was that alignment of the IF amps was fairly critical, and that a good instrument for doing that job is a wobbulator? I do have a 10.7MHz wobbulator, bought some years ago from a friend, but I've never actually used it.In terms of FM tuners, I still have a ~35 year old NAD 7020 in daily use, but I'm pleased to say that I've never had to do any service work on that.Incidentally, are you familiar with this UK forumhttps://www.vintage-radio.net/forum/index.php. There are some very knowledgeable devotees of FM radio on there; DAB is a swear word.

author: djsb
author_rank: Super Contributor
timestamp: October 07, 2021, 04:40:53 pm
content: I follow and post occasionally on the vintage radio forums and yes there are some very experienced people on there. I'd like a low distortion oscillator, as the alignment procedure for my NAD 4300 FM tuner involves measuring the distortion level. I'm hoping to get a Quant Asylum QA402 Audio Analyser at some point to do the actual distortion measurements. I'm also working on a Revox B261 tuner at the moment (mainly recapping, and I've replaced the relay on the audio PCB with a custom PCB with Axicom IM07 relays as the OEM relay is unobtainable. It's contacts were intermittent anyway).

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2021, 05:10:26 pm
content: The FM signal often uses quite nonlinear amplifiers in the receiver to limit / saturate the amplitude. So the IF signal itself is not expected to be very low distortion. With the VCO one may want a relatively linear frequency modulation input, so a linear control voltage. This depends on the curve if the varicaps.Another point to whatch is that the amplitude is not effected very much from the FM modulation. So the tuning voltage should not effect the amplitude very much (the direct effect looks good in this configuration). A relatively fast amplitude regulation loop can also be good. If needed, one could consider adding some of the tuning voltage also to the amplitude control part to add some feed forward correction if a lot of correction is needed.