UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 6-gang FM stereo tuner heads

regenfreak · 30th Oct 2022, 12:08 pm

I received the Tek P6202A and pogo test pins this week. Last night I homebrewed a miniature power plug for P6202A -15V, +15V input socket and powered it up with the Farnell Triple Tops 2D power supply. It is working. I put the ground spring pin from my Rigol scope probe into the tip to eliminate the need of a ground lead.

The gain is very flat when i tested with my short transmission line, however, the results vary depending on whether I terminate the line with a 50-ohm load or not:

gain = -23db with 50 ohm termination of the transmission line
gain = -17.8db with open line

Furthermore, i measured the gain of P6202A with a FY6900 signal gen and a 200Mhz scope at 100MHz (with 50 ohm input). The measured gain is -20db or 10:1 voltage ratio which is the manufacturer's spec. Therefore, the actual gain is somewhere between the open and 50 ohm terminated transmission line measurements. This throws up my long-time question of the measurement of high impedance DUT with large mismatch uncertainty.

I have tried to measure the Rp, Xp of the P6202A (attached) I believe the service manual of the P6202A is incorrect to quote Rp >100k from DC to 500ohms. The Rp should decreases sharply with increasing frequency based whatever small-signal, high frequency model of FETs.

I probed the IF output of FM tuners with P6202A and never get anything like this guy got with the P6202A and HP spectrum analyzers:

http://ham-radio.com/k6sti/swept.htm

I am a bit flabbergasted; he got nice response bandpass curves using the "residue sample at the IF output" while I got flatliner on the spectrum analyzer....Have i missed something?

As expected, the P6292A has greater probe loading than the Poorman's probe due to high input capacitance (about 1.8pF). It loads down the 10.7NHz IF strips greatly, flattening the Qs of the passband filters.

I have attempted to measure the gain of the Poorman probe with a 0.3PF air gap gimmick C1 (.measured at 100kHz with DE-5000). I have used pogo pins for both the probe tip and ground tip. Its performance in term of the flatness of S21 is rather dismal (see attachments). The gain is below -40db (equivalent to 100:1 probe). Note that it has become noisy and difficult to get repeatable S21 measurement even I added a 30db LNA to amplify the signal. The NanaoVNA V2 Plus 4 has a dynamic range is close to 90-100db with averaging.

So I have gone back to Porrman's probe with higher input capacitance (C1 = 0.79p measured at 100kHz with DE-5000), its S21 flatness and gain are more acceptable. Here i have attached the equivalent parallel and series resistance; Rp, Xp, Rs, Cin and ILs. They are crude measurements really but the order of magnitude seems to be OK.

30th Oct 2022, 12:08 pm	#198
regenfreak Heptode Join Date: Nov 2018 Location: London SW16, UK. Posts: 655	Re: 6-gang FM stereo tuner heads I received the Tek P6202A and pogo test pins this week. Last night I homebrewed a miniature power plug for P6202A -15V, +15V input socket and powered it up with the Farnell Triple Tops 2D power supply. It is working. I put the ground spring pin from my Rigol scope probe into the tip to eliminate the need of a ground lead. The gain is very flat when i tested with my short transmission line, however, the results vary depending on whether I terminate the line with a 50-ohm load or not: gain = -23db with 50 ohm termination of the transmission line gain = -17.8db with open line Furthermore, i measured the gain of P6202A with a FY6900 signal gen and a 200Mhz scope at 100MHz (with 50 ohm input). The measured gain is -20db or 10:1 voltage ratio which is the manufacturer's spec. Therefore, the actual gain is somewhere between the open and 50 ohm terminated transmission line measurements. This throws up my long-time question of the measurement of high impedance DUT with large mismatch uncertainty. I have tried to measure the Rp, Xp of the P6202A (attached) I believe the service manual of the P6202A is incorrect to quote Rp >100k from DC to 500ohms. The Rp should decreases sharply with increasing frequency based whatever small-signal, high frequency model of FETs. I probed the IF output of FM tuners with P6202A and never get anything like this guy got with the P6202A and HP spectrum analyzers: http://ham-radio.com/k6sti/swept.htm I am a bit flabbergasted; he got nice response bandpass curves using the "residue sample at the IF output" while I got flatliner on the spectrum analyzer....Have i missed something? As expected, the P6292A has greater probe loading than the Poorman's probe due to high input capacitance (about 1.8pF). It loads down the 10.7NHz IF strips greatly, flattening the Qs of the passband filters. I have attempted to measure the gain of the Poorman probe with a 0.3PF air gap gimmick C1 (.measured at 100kHz with DE-5000). I have used pogo pins for both the probe tip and ground tip. Its performance in term of the flatness of S21 is rather dismal (see attachments). The gain is below -40db (equivalent to 100:1 probe). Note that it has become noisy and difficult to get repeatable S21 measurement even I added a 30db LNA to amplify the signal. The NanaoVNA V2 Plus 4 has a dynamic range is close to 90-100db with averaging. So I have gone back to Porrman's probe with higher input capacitance (C1 = 0.79p measured at 100kHz with DE-5000), its S21 flatness and gain are more acceptable. Here i have attached the equivalent parallel and series resistance; Rp, Xp, Rs, Cin and ILs. They are crude measurements really but the order of magnitude seems to be OK. Attached Thumbnails Last edited by regenfreak; 30th Oct 2022 at 12:36 pm.