KitsAndParts.Com Forum

You can see my evaluation and explanation of the mixer at http://golddredgervideo.com/kitsandpart … dmixer.htm
Leonard

wa2me wrote:

What changes would I have to make in the diplexer for a 9mhz IF?

The simplest way is to rewind the inductors; use 10 turns on the FT37-61 and 12 turns on the T37-2.
-Diz

There are two attenuators  on the board, one controls the oscillator injection level, the other the signal input level.  It's probably a good idea to have the signal input attenuator off board so you can bypass it as needed.  The signal input attenuator's purpose is to keep from overloading the mixer from strong signals.  Most rigs will have the option of switching in either an attenuator (or several for different levels) or a preamp depending on band conditions.  The oscillator attenuator can be left on the board and the resistor values depend on the output of your vfo / dds unit.  Attenuators also provide a fixed input impedance to the mixer which is desirable, so if you stuff the board with attenuators having low loss (1-3db) and make up the loss with a pre-amp / osc driver the mixer will perform better.  Note that there is some bad effect on mixer noise figure from having the input attenuator (but an overloaded mixer is WORSE than the input noise, and below 30mhz antenna noise dominates over mixer noise anyway).

Say DIZ, why is the design of the diplexer on this asymmetrical with two different inductors?  Most of the diplexers I've seen following mixers had identical L and C for both the series and parallel elements.

Being a newcomer to the double balanced ring diode assy----What equipment is necessary to adjust the 2 trimmer capacitors in the diplexer?  What am I to to look for by tuning each of them? Specifically, what is the purpose of each?    73

W8DIZ wrote:

QRP90MW wrote:

Being a newcomer to the double balanced ring diode assy----What equipment is necessary to adjust the 2 trimmer capacitors in the diplexer?  What am I to to look for by tuning each of them? Specifically, what is the purpose of each?    73

Need to adjust both trimmers for maximum output signal at the IF.
Both caps are part of an LC tuned circuit and should be adjusted to the IF center.
-Diz

That will get you in the ballpark, if you have the right test equipment you can get it closer to the ideal alignment which will give the lowest swr (best match to 50 ohm output) and best intercept figure.  I think the procedure was described in a recent ARRL HB.

hi. i have a question related to QRP90MW's question. what is the tuning range for the diplexer?


kc0wox's evaluation article shows a IF freq of 5MHz. i want to get a IF of 455KHz. will the diplexer work at that low a difference frequency?


i was also curious if this mixer would be suitable as a modulator for a qrp am transmitter?

Last edited by ssbothwell (2011-09-24 02:47:54)

is the diplexer by default setup for a 5MHz IF freq? what is the advantage of one difference frequency over another? i was under the impression that 455khz is fairly standard for superhet receivers.

as far as using it as a modulator, i would want to bypass the diplexer right? how much power do you think i could safely run through it without damaging the diodes?

thanks for the comment.

okay good to know it is setup for 5MHz. thanks for the info. i'll report back once i have tested the mixer as a modulator.

ssbothwell wrote:

i'm having trouble getting finding the IF frequency. i a using a 3.6MHz ~9dBm hartley oscillator on the RF input and an HP3312A function generator set to 8.6MHz (RF+5MHz) on the LO input.
on the IF port i am seeing an 8.6Mhz signal. i have tried adjusting the two variable capacitors but they dont seem to make a noticable change. any ideas what might be wrong?

The mixer is a BALANCED mixer, hence the LO signal should be down 30-40 dB. Sounds like you have an unbalanced condition...maybe a bad solder joint on the mixer IC? -Diz

Hi,

Isn't there an error in the labelling of J2 and J3.
J3 on the PCB board is J2 on the schematic and J2 on the schematic is J3 on the board.

Michel ON4DCP

hi w8diz. i have been really busy lately and have not had a chance to work on this mixer recently but i finally got a chance to take it off the shelf and mess with it today. i am still not able to make any progress.

like i said, i am using a fixed oscillator at 3.9MHz 9.5dBm and an HP-3312A function generator for LO and RF (i have tried them both in both positions). there definitely seems to be balance issues because i see the function generator input in the IF port. however, i have checked all my solder joints several times and cant figure out what is wrong with the circuit or why it is behaving unbalanced.

i really want this thing to work.