[HBQRP] FW: [pixie2] Digest Number 258

[Piatt, Darwin]

At one of our get together events, some of us were discussing using the SDR software with the little Pixie and other DC type receivers (like the Sudden).
Here is a train of messages from the Yahoo Pixie 2 group that discusses doing this, read on, you might find a new toy!!!!

I downloaded the 'recommended' software last night but the stuff folks downloaded for the little SDR kits works fine also. I will have a demo of this at the next group meeting.  Ain't computers fun?

Darwin (Dar) Piatt - W9HZC
Midwest HomeBrewer's and QRP Group
http://www.qsl.net/hbqrp 

p.s. I've cleaned it up a bit so you can see the real stuff, anyone tries this, you should report back to the group, of course!
73's - Dar..
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From: pixie2 at yahoogroups.com [mailto:pixie2 at yahoogroups.com] 
Sent: Monday, December 15, 2008 11:36 PM

Set Your Pixie Free!! From: Bill Stevenson
 
1a. 
Re: Set Your Pixie Free!! 
Posted by: "Bill Stevenson" g4kki.william at ntlworld.com   sparks98uk 
Mon Dec 15, 2008 5:52 am (PST)
 
Stephen,
amazing, i put the pixie on last night and used that M0KG software
and i tuned up and down 40 mtrs listening to CW with it. I dont know how it
compares with a SDR receiver but it was doing a pretty nice job here. 
For someone who wants a simple little cheap receiver to listen to a ham band on I don't think it would come much cheaper......
Bill
G4KKI

Stephen Wandling wrote:

 I posted this on the QRP-L list yesterday and thought some of your might
 find it interesting.
 ========================================

 Actually, its Michael Rainey's fault. You will recall that awhile back
 he had a great post here (qpr-l) about the "QRP Minimal Art Session", a
 neat contest where contestants participate with rigs having a very low
 parts count. He went on to mention some of the rigs that have been used
 and mentioned the Pulga, aka The Flea.

 The Flea is the work of a Catalonian amateur radio couple, Joan and
 Eduardo, ea3fxf/ea3ghs. There wasn't a lot on the Internet about this
 little rig, but I found enough to get quite interested. They had posted
 a Spice model, one showing the transmit function, and a model showing
 the very high selectivity of the "filter" that's involved with the
 receiver. I was hooked.

 At that time, I was committed to my Pixie education project: Learn to
 model it in Spice, do the prototype board build and try mods, and
 finally put it on a piece of pcb. Recently I finally got it into the
 "ugly" format and was checking it out. Rigged as a "Super VXO" with two
 identical crystals in parallel and a series variable capacitor it would
 tune 7038.5 to7044.5 Khz. 6 Khz seemed like pretty good range for such
 a simple rig. But, my thinking would soon change.

 I don't recall where I saw a mention that the Flea was being discussed
 on the EMRFD Yahoo group. Having recently bought the book and not
 knowing about the group, I joined. I posted a slew of questions and
 Joan and Eduardo answered right away, as they sat together having their
 tea. They loaded me up with info, a link to a 'calculator' that they
 use, and ultimately with the urge to ask even more questions. But,
 first I would check out their info.

 The first was that they could copy signals 24 Khz above and below their
 Flea receiver's frequency. They observed this using M0KGK's SDR
 Decoder. Now, I've never gotten too excited about Software Defined
 Radio, as it seemed to be a step towards complexity, whereas I have a
 tendency to head in the other direction. Simplicity.

 But, eventually I downloaded the free software, and after struggling
 with the learning curve, I got it matched to my computer's sound card
 and could get an output. Let's jump to today.

 Today I thought "If they can input the audio from their Flea, why don't
 I try inputing the audio from my Pixie?" So, with a simple cable from
 the Pixie to the Line In jack on the sound card, I was in business.
 Using my new FCC1/FCC2 I cranked the frequency up as far as possible on
 the Pixie and measured 7044.5 Khz, which I entered into the SDR
 software. And now I was looking as at a spectrum of noise with spikes
 on it. I selected CW and clicked on one of the bigger spikes and could
 hear CW and the software gave me the frequency of the signal. I started
 clicking around on all of the spikes and realized they were each a signal.
 It was only when I was playing with using Page Up/Page Down to quickly
 tune across the band (Left/Right arrows to fine tune) that I realized I
 could get the whole screen to scroll as I kept pushing the Page Up
 button. Soon it stopped and I was seeing frequencies above the 7066 Khz
 range. So, I tried the other direction and I could see signals below
 7023 Khz. Amazing. And I hadn't touched the Pixie, which was still
 tuned to 7044.5 Khz.

 Now, I could go on about having access to 5 CW filters from 500 Hz to
 100 Hz, or the AGC, RIT, S Meter, frequency readout, notch filter, etc.
 It was truly amazing. In fact, I'm still staring at the screen as I
 type this. But, its ignorance time folks.

 How is this happening? I've done a bit of reading about SDR, quadrature
 mixers, "I" and "Q" outputs 90 degrees out of phase, and all that jazz.
 But, the Pixie does not generate I & Q signal streams. It's just
 audio. The only RF info it gets is when I manually input the frequency
 that my Pixie is tuned to. From there the SDR software generates the
 44 Khz band of signals that I am receiving from the Pixie.
 I can assure you that when I put on the headphones and listen to the
 Pixie its no where near as impressive. I can generally hear a couple of
 small signals and that's about it.

 So far, the only correlation that I have found is that the 44 Khz of
 passband corresponds to the sound card's sampling rate of 44.1 Khz.
 This from reading Software-Defined Radio for the Masses.

----------------------------------------------------------------------
1b. 
Re: Set Your Pixie Free!! 
Posted by: "Kenneth Stringham" ae1x at yahoo.com   ae1x 
Fellas,

Let's remember that the Pixie2 has little or no filtering. The only filtering involves what we do during our receiving capability. The limitation, then, is the sound card capability.

Ken - AE1X
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1c. 
Re: Set Your Pixie Free!! 
Posted by: "Stephen Wandling" swandling at gmail.com   magicod 
Mon Dec 15, 2008 10:31 am (PST) 
Hi Bill,

Yes, after you've used your Pixie with headphones or a speaker and then 
connect it to SDR software, the difference is truly amazing. One of the 
first things that came to mind for me is that with the SDR capability 
and its RIT function, this was more like the first station I built and 
operated. I had a crystal controlled xmtr and a military surplus 
receiver. I would call CQ, listen for a bit on my frequency and then 
start tuning around to see if another crystal controlled rig was 
answering me.

The Pixie + SDR does have a few issues as you probably know. In the SDR 
configuration, the Pixie can be seen as the Local Oscillator. Let's say 
you set the Pixie on 7040 Khz and you are receiving a station on 7030 
Khz, you will also find its 'image' signal on 7050 Khz. This is a 
"feature" of direct conversion receivers. If you're just being an SWL, 
its not much of a problem. But if you were using it as your station 
receiver and had another xmtr with a VFO, you would have to figure out 
which signal to respond to.

Steve "Melt Solder" Weber, KD1JV, posted a response to my QRP-L post, 
and gave a method of sorting out the signals from the images. If you 
have a way to shift your Pixie crystal frequency, such as a series 
capacitor, if you shift the Pixie higher in frequency, one of the 
signals will move higher in frequency and the other will move lower. 
The one that moves opposite to the shift in Pixie frequency is the 
signal and the other is its 'image'.

Tied to the image issue with direct conversion receivers is that you can 
get a signal on 7030, and at the same time get a signal on 7050. I had 
this happen yesterday. There was a guy blasting away at 30 wpm on the 
lower frequency and another guy on his higher, 'image', frequency 
sending at about 7 wpm. It was only when I shifted the Pixie frequency 
did I see these two signals move apart and both became copyable. 

I have a question for you. On the M0KGK software, I see two traces, one 
brighter than the other. The fainter image is slightly above the 
brighter one. There seems to be some correlation between the two, with 
the fainter one seeming to 'lag' the brighter one. I don't see two 
spectrum in the screenshots from the Flea transceiver people.

Speaking of the Flea (Pulga) transceiver, its very similar to the Pixie, 
but different. Just one transistor and an Op amp. They have used it to 
copy PSK31. I haven't tried this, but will be modifying my Pixie to 80M 
where I will have access to PSK signals and will check it out. 

They are very creative and interesting people, and have lots of simple 
ideas that I will enjoy playing with. If anyone is interested to learn 
more about the Flea and their other projects, I can post some links.

72
Stephen
VE7NSD

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1d. 
Re: Set Your Pixie Free!! 
Posted by: "Stephen Wandling" swandling at gmail.com   magicod 
Mon Dec 15, 2008 12:02 pm (PST) 
Bill,

Regarding my question to you about seeing double spectrum, I have 
figured it out. The faint image is the "hold" image. Going to 
Options/Receiver/Display/Hold Time and setting it to zero gives just one 
spectrum. Phew!

72
Stephen
VE7NSD

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