Some home grown measurements

Not perhaps very accurate perhaps, but a few small boxes of tricks can make home brewing and measurements possible.

Oscilloscope - who would be without one? I use a LabNation USB interface scope with two channels, with software running on my MacBook, seems to be enough for my audio to HF needs.

Oscilloscope

Signal generator - I would not be without my general purpose signal generator built with an Arduino Nano driving an Si5351 synth chip (and it has a RTC built-in too). Programmable!!! can be used for many applications. A VFO from <1MHz to > 150MHz, with the possibility of up to 3 outputs. Another use is for generating RF signals with 0/90deg phase shift for experimenting with phase method SSB RX & TX designs. And then again it can generate CW for beacons or training and QRSS, WSPR transmissions. The only thing to recognise is that the outputs are square wave, not pure sine wave. The output is +12dBm (860mV , 15mW into a 50R load).

Signal Generator

Power or RF voltage meter - Based around an AD8307 chip this RF meter has a high impedance or a 50R dummy load input. As the AD8307 is a log amplifier it has a  huge range of -40 to +40dBm (1uW to 10W!). 

Three little boxes - a 7MHz low pass filter, a 20-10-5dB attenuator and a 30dB tap with 50R dummy load built in. 

Top :  LPF, Centre: 20-10-5dB attenuator, Bottom: 30dB tap with 50R dummy load

The 30db tap is particularly useful for checking > 10W power inputs to the dummy load, giving a -30db output to a second RF Meter I have with a direct AD8307 input capable of  -70 to +20dBm with a 50R input impedance. It is also useful the other way round.

Power amplifier - one thing I have missing is a small power amplifier, with say up to 10W +40dBm output from a 0dBm input, input and output 50R. I am working on this design right now as a project to better understand medium power PA stages and preamplifiers.

MY SHACK

13.8V PSU, 

GPS clock & QCX+, 

paddle  antenna switch

and ELAD FDM_DUO transceiver

The RF Signal amplifier

 I was talking to another amateur the other day, about a preamp stage for a xtal filter and a following amplifier stage. The xtal filter needed to see a source impedance of 250R and a load of the same.

Now the basic transistor, common emitter, transistor and equivalent circuit looks like this

Where the collector is a current source/sink. A complete circuit would look like this

Transistor AC gain is B

Where Rs is the input source impedance, RBIAS is the bias resistor(s), re is as above, Rc is the collector load (which determines the output impedance) and RL is the load (for maximum power transfer Rc = RL).

Using this circuit we have

Gain Av = Vout/Vin = Rc * ic / re * ie, 

  and since ic ~ ie, Av = Rc/re

Input impedence Zin = re*ie/ib = re*B (when RBIAS >> B*re and B >> 1)

Zout = Rc

So an amplifier with 50R source impedance and 250R output impedance (for the xtal filter source impedance) would look like this

Amplifier

Assuming

B = 100

Ic = 10mA

re = 26/10 = 2.6R

Transistor Zint = 26/10 * 100 = 260R

Amplifier Zin = 50R with a 1:3 transformer

Amplifier Zout = 250R

The transistor bias can be arranged however you want provided the bias resistors are large compared to Zint.  

The output of the xtal filter also needs to see a load impedance of 250R, but this about the same as our amplifier input impedance (260R), so we could have this simple design

Xtal filter post-amplfier

The output side of the post-filter amplifier can be whatever is needed for the following circuits.

But the input impedance is still critical on the collector/emitter current and on the AC Beta (gain) of the transistor. Chosing Ic = 10mW has determined the input impedance.

Feedback

So the next thing to look into is building a feedback amplifier where it might be possible to determine the input/output impedances using some sort of negative feedback across the transistor. And so it is.

This is a simple amplifier (sorry poor screen grap from youtube...)

You can see

1. A 1k feedback resistor, meaning with 50R input source, the gain is Rf/Rs = 1000/50 = 20.

2. The bias is set with the Ve = 3V, and bias is set by the 470R resistor, which does not affect the source resistance much

3. The amplifier gain has to equal the the feedback gain, i.e. 20. This is determined by the RL/Re. RL is 200R (chosen so that a 1:2 transformer drives a 50R output load). And so Re has to be 200/20 = 10R.

Two radios need an antenna switch

Foolish I know, but if you have two radios then you need either two antenna or an antenna switch! I was fed up with unplugging one and plugging the other so I made this

The antenna switch and

my QCX+ and FDMDUO TRXs

The unselected radio is terminated by a 51R resistor, just in case you accidentally put it into TX mode!

Morse?

I am using the QCX+ at the moment to learn morse code - with the help of a random letter sender based on an Arduino Nano and a Si5351 sythesiser, transmitting on nominally 7023kHz (CW_LEARN code here). The QCX+ is tuned to 7023.0 after calibration using a GPS NMEA & PPS input (Menu 8.11 & 12).

By the way the "clock" (see pic above) under the QCX+ is my GPS clock, used also to provide NMEA and PPS signals to the QCX+. The GPS operates at 3V3, so I use a 3V3 <-> 5V converter to drive the QCX+.

For fun

I am playing also with a Pixie TRX, also tuned to 7023kHz and picking up the Arduino morse tutor.

The Pixie $5 TRX!