Sunday, December 05, 2010

FT-102 IF output buffer

The Yaesu FT-102 has a lot of outputs/inputs in the back pannel and a lot of controls in the front which is always nice....
Two of the most interesting outputs in the back are the 8.2Mhz and 455Khz IF out marked as 2º and First respectively (strange!?). The 455Khz one is filtered so you don't get many signals outside the pass band but the 8.2Mhz one is just before the 8.2 cristal filter in the FT-102 giving you an overlook on signals after the first mixer. That could be interesting to place an panoramic adapter or a second receiver. I decided to see (level wise) what signals I get there. I you connect directly an HF receiver tuned at 8.215Mhz you will get the same signal that the radio outputs by the speaker (at a much lower level though) also if you go out 10Khz from 8.215Mhz you are listening with 10Khz offset, logic in all similar concept receivers! You get the point, else you were not reading this. The limit is only the passband on the input filters, but they are built to give the entire band.

On to the schematic...

The bufer (from FET pricples and circuits part 2):

The 6K8 on the output was replaced by a 150 Ohm one
The input 220n was replaced by a 100n one.
The buffer output (out) connects to the the amplifier block (bellow) on point 1, point 4 (bellow) connects to the HF receiver tuned at 8215Khz (or the offset you need)

R is 220 Ohm and C is 100n (tipycal....) transistor is 2n2222, VCC is 12V, remove the 1n4148 diode (replace by a shunt! no need here). This is a block from "bitx" transceiver and works nice.

Those 2 blocks allows to increase the signal level to the point that on a normal HF receiver (TS-50 tested) the signal of the signal meter is more or less the same that the FT-102 is displaying. There are still things to improve like puting a 8.2Mhz "band" wide filter between the 2 blocks or a tuned rf transformer from the radio to the buffer stage.

My idea is to put an panoramic receiver after this buffer/amplifier or connect to a scope for in band signal monitoring.
During tests a single downconverter with an NE602 was conected directly at the amplifier stage output but the NE602 is noisy as hell (or was not well coupled) so will probably replace for something else in the future. Anyhow this was just a proof of concept.

The buffer idea come from here:

No comments: