EMRFD Message Archive 969

Message Date From Subject
969 2007-08-23 08:18:35 kc0vkn Buffer stage troubleshooting
Good day to everyone,

Even though the intro says that EMRFD isn't a list of projects to be
completed, I've taken it upon myself to start building some of the
circuits in an attempt to learn whats going on, and learn how to
utilize my test eqiupment.

I've got the basic transmitter circuit running and I'm fiddling with
the first buffer stage because power output seems quite low.

The oscillator seems to put out about 2.3v PtP, but once I add in the
buffer stage I'm not seeing quite as much power on the secondary side
of the transformer as it seems that I should. I see about 1.2v p2p in
my scope, or about 3mW of output.

I've got a 50ohm load tacked on the output where I'm pulling it off
10x to my scope. I've also run it into the 50mW power meter described

I'm trying to sort out where I may have made an error, and what to
walk back through.

If I take the buffer stage off and check output from the base of Q1, I
see a very clean sine (and a bit more than 2V PtP). Once I start into
the buffer, the sine on the primary side of the transformer also looks
good. When I get to the other side of the transformer, it's not so
perfect looking anymore... I did recently see a discussion attribted
to W7ZOI about "perfect sine syndrome" when folks have a scope at hand!

I didn't have to change any parts from the schematic, save Y1, of
which I only had a 7.200 xtal. Circuit oscillates at 7.199.5, so, it's
happy, just a bit weak.

I've fiddled around with the emitter loading on Q2 and found that
lowering the value increases my output, but, I'm not sure if thats
wise, given that my initial output power deviates substantially with
what I'm thinking I should see.

I must admit I'm pretty green, and I suspect I'm at the cusp of a
grand learning experience--and I'm missing something I should know, or
do know and don't know how to apply it. I hesitated posting the
question as most of the other discussions seem far in advance of my own!

Does anyone have some tips?


972 2007-08-24 08:03:23 kc0vkn Re: Buffer stage troubleshooting
981 2007-08-26 21:03:35 Wes Hayward Re: Buffer stage troubleshooting
Hi Joe and gang.

I'm pleased that you are using that First Transmitter from Chapter 1
of emrfd. Quite a few folks have been doing the same thing. To
assist in this effort, I went through the original and did a bunch of
measurements. The following data was gathered:
1) DC voltages through the first three stages, measured with no
RF. The crystal was temporarily pulled out.
2) The DC output of the RF Probe of Fig 1.29, measured with a DVM.
3) The peak to peak signal in a scope, measured with a 10X voltage
attenuator probe.

I now have this data up
985 2007-08-27 12:50:06 kc0vkn Re: Buffer stage troubleshooting