if tap - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: January 16, 2024, 08:28:46 pm
content: can anyone tell me the best place to tap the if on an old yaesu707 for a panadaptor?,cheers 73

author: Andy Chee
author_rank: Super Contributor
timestamp: January 16, 2024, 09:02:07 pm
content: My suggestion is here:http://www.foxtango.org/ft-library/FT-Library/Eighties/FT-707_serv_man.pdfThe tap connection should be relatively easy to find because there appears to be a connection between two circuit boards.I would preferably use another buffer from the tap point to drive your SDR.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 19, 2024, 10:47:17 am
content: thanks andy 73.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 20, 2024, 01:36:01 pm
content: any idea of what buffer i need and how to build one,schematic etc?.

author: A.Z.
author_rank: Super Contributor
timestamp: January 20, 2024, 03:53:22 pm
content: https://www.vintage-radio.net/forum/showthread.php?t=179135the above should give you an idea, then there are many ways to skin a cat

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 20, 2024, 09:28:56 pm
content: Quote from: p.larner on January 20, 2024, 01:36:01 pmThe caps are to block DC, choose a value big enough for the lowest frequency of interest, the voltage divider sets the bias point of the transistor and as long as it is a bit less than 1/2 VCC it should be fine, but shoot for 1/2 vcc - the voltage drop from base to emitter, the emitter resistor sets the current through the transistor, say something popcorn like a 2n3904 and you want say 10ma current. Essentially you want a really high input impedance and a really low output impedance to isolate the tap point from your panadaptor circuit. There is a bit more to it than that, but its a pretty forgiving circuit. You should be fine.I dont know the frequency of your IF out, or what voltages you have easy access to or how high an impedance is needed to take it much farther than this, or what the signal voltage is out of the IF tap is, you might need to add a pad on the output to tame it down for the input to the pan adapter.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 21, 2024, 08:21:53 pm
content: Its for my FT-707,as far as vcc goes i have whatever is in the radio.It has 12v and 8v rails mostly if i remember,73.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 22, 2024, 09:14:24 pm
content: Quote from: p.larner on January 21, 2024, 08:21:53 pmStill cant do anything without knowing what the IF frequency is. I dont know that radio, im assuming its either 10.7meg or 455khz, but have no idea really.There is another option, using an opamp, this requires less calculations. Just choose an opamp that has a gain product bandwidth a few times bigger than the IF frequency and it should be ok, like 5 times. All the caps can be 100nf and the resistors can be all 100K. The trade off is high speed opamps cost a lot more than doing some math for a popcorn transistor.But in the BJT circuit, the OHMS law for the emitter resistor, we know V = 8 and we know I = 0.01 solve for R = 800, the voltage divider circuit Vout = Vs * R2 / R1 +R2, Vs = 8 R1 make 10K Vout is Vs / 2 - 0.7 so 3.3V solve for R2 7K ohms. The caps without doing math for 10.7Meg 100nf, maybe 1uf at 455khz.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 22, 2024, 10:17:14 pm
content: it saysReceiver system: Triple conversion superheterodyne1st IF: 8.99 MHz2nd IF: 10.76 MHz3rd IF: 8.99 MHzDoes that help?,thanks+73.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 22, 2024, 11:23:05 pm
content: Quote from: p.larner on January 22, 2024, 10:17:14 pmYeah design the buffer as if the tap is 10.7Meg, either or that I posted above should work.

author: RFDx
author_rank: Regular Contributor
timestamp: January 22, 2024, 11:37:12 pm
content: Attached are two buffer variants with a JFET for 8V or 12V supply. Gain is ~0.5 (-6dB) in both cases. Inputs are high-Z and outputs are 50 Ohm. The reverse isolation of the upper circuit is better than 40dB and that of the lower circuit is better than 30dB up to 30MHz.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 23, 2024, 11:19:51 am
content: could a bf981 dg fet be used in that cct?,i have some in my junkbox,cheers+73

author: RFDx
author_rank: Regular Contributor
timestamp: January 26, 2024, 02:10:38 am
content: Quote from: p.larner on January 23, 2024, 11:19:51 amIn the first buffer example replace the JFET with BF981 DG-MOSFET, tie gate 2 to drain and lower the source resistor to something like ~22...33 Ohms to get ~50 Ohm output impedance.