Note T6 should be connected differently http://sites.google.com/site/g4zfqradio/softrock_ensemble_isolation_transformers
MY QUOTED MEASUREMENTS MUST BE TAKEN AS VERY APPROXIMATE, ERRORS MAY BE PRESENT. THERE MAY WELL BE INCONSISTENCIES BUT READINGS SHOULD BE IN THE GENERAL RANGE
I decided to record these measurements from a working Softrock. I actually used a v 6.1 but they all have similar circuits.
I used IQout http://www.m0kgk.co.uk/sdr/download.php to produce an IQ signal.
any SDR TX program may be used but this makes it easy to set.
These are my settings. I chose a fairly low frequency as many digital meters on the AC range will work at audio frequencies.
Select your soundcard and drivers in use.
Connect the Softrock TX input to the Line Out.
Connect a power meter and dummy load to the Softrock output.
Power up the Softrock and USB. GFGSR may be used to set the frequency to the lowest range covered. The latest version http://home.ict.nl/~fredkrom/pe0fko/CFGSR/ has buttons to make band changes quick. AND on the "Test" Tab it has a PTT button!
The Ensemble does not have COM port PTT so you can use a cliplead to ground the top left-hand lead of U4 (Pin 4), activating PTT. Thanks for the pictures Robby! BUT see CFGSR above! http://www.wb5rvz.com/sdr/ensemble/index.htm
TOOLS:- The AC range of many modern digital meters is specified to work at audio frequencies. If you have one of these then it will probably tell you the approximate audio RMS voltage present.
For the RF measurements this can not be used. An oscilloscope or just a simple diode probe will give a good idea of the levels present. The oscilloscope and it's probes will need to be specified to work on the test frequency. A diode probe should cover any HF frequency.
Diode probe example http://www.n5ese.com/rfprobe2.htm (Lots of things on this site!) This will read RMS with a 10M Ohm digital meter. Multiply by 4 if you have a 1M Ohm meter.
Below, my build of one of these.
A piece of copper clad board cut to fit inside a pen shell. Lands made with small saw blade.
It is a single diode probe with a low-value resistor, not critical, 15K used here. It will read PEAK voltage.
A complete unit. This one is an integrating probe, a series 47K resistor followed by a 0.1uF to ground. The red clip is the ground connector, I'd run out of black sleeves! This may be used for measuring DC voltages where RF is present, like on the Softrock divider and mixer.
AUDIO FREQUENCY TESTS
Set IQout "Volume" to a low setting, start it and enable PTT. Gradually increase this level until 500mW (1/2 watt) output is achieved. 500mW is 5V0 RMS, 7V1 peak measured at the antenna terminal with the dummy load in place. If this is not reached I hope the measurements will give a clue where to look!
Set the audio to my measured level and see where they vary greatly from mine, below.
I have taken the schematic from Robby's SDR site. http://www.wb5rvz.com/sdr/ The red dots show where the exposed "hairpin" of the resistors are.
Audio voltages may be measured at the hairpins of R26, 25, 28, and 27.
For 500mW out I measured about 1V2 peak-peak on an oscilloscope and about 0V5 RMS with a digital meter.
Voltages at pin 2 and at the audio inputs will be similar.
If the input voltage is lower than this then check the soundcard mixer settings.
Some cards will go much higher,
PEAK TO PEAK. Make sure the oscilloscope and probes are specified for the frequency used. Frequencies higher than this may be used but sensitivity will be less and readings lower.
Normally a x10 probe will be used to obtain the specified frequency response. Check your measurements using the scopes "Calibrate" signal or put it to DC input and check the 5V line or a battery to be sure you are reading correctly.
Transformers will show a reading on three of the six connections, at one end of each winding..
One of my diode probes measures approximate PEAK voltage the other RMS. [RMS probe 0.7 times lower voltage. These RMS voltages were confirmed using a Bradley RF voltmeter with an unterminated probe] I found my RMS diode probe gave unreliable results at T2 where voltages are low.
Tests at 3.5MHz.
These points are easily identified underneath the board, not so easy from the top but at least some of them are accessible. You do not need ALL these measurements.
All measurements need a fine probe and care must be taken not to slip and short two points together.
NOTE Some of these readings do not seem consistent. Take them as approximate. The important thing is that they should increase as you progress through the transmitter.
I did some tests on my Softrock 6.3, which is really the same as an Ensemble but with plug-in coils. This seems to show that the Softrock TX is limited on the high bands.
These are derived from the amount of drive required using WSPR. This has a drive control calibrated in dB which seems reasonably close. (I measured -29db over it's range of -30dB, most likely the 1dB difference is my error.)
Using a fixed audio level I recorded the following WSPR attenuator values.
I measured at the equivalent points below. Again the measurements from the probe, probably a different one than used above, do not match the scope readings at low voltages.
Test at 28MHz.
Driven to about 500mW on 28MHz. The audio drive was 1V8 peak to peak on the scope.
You will realise that a diode probe is ideal for setting low TX powers. One may be made with the addition of a 50 Ohm load with a coax lead for the Softrock output. The dummy load may just be a 1/2 watt 50 Ohm resistor or a parallel combination. Even 4 off 220 Ohms and a 560 Ohm in parallel would suffice.
My method of checking the 1dB compression point.
This is a test to determine the maximum linear output of an amplifier. An oscilloscope will not show small levels of distortion.
I measure the RF output across a 50 Ohm load reasonably accurately. I use an old Bradley RF meter, convert to dB.
A diode detector, like the probe above should measure PEAK RF voltage with a 10M Ohm meter. But check this. Calibration may still be necessary.
There are lots of conversions two here http://www.tecmag.com/pdf/dbm_v.pdf Peak V = dB = mW
http://www.eeweb.com/toolbox/rf-unit-converter Enter a figure, the rest is calculated.
Using something like IQGen (see above) I increase the drive dB by dB. As power increases there will come a time when the output does not rise a whole dB. When the RF output lags the input by 1 dB then that's the 1dB point.
This occurs at something like 1 watt on some low bands on some of my RXTXs but not that much in the mid-high bands.
Then a scope may be used to check the level. Do not exceed this level for linear modes like SSB, PSK.
Warren's way of checking linearity with a second Softrock http://groups.yahoo.com/group/softrock40/message/70260
And Ed's way using a scope. http://groups.yahoo.com/group/softrock40/message/70268
G4ZFQ February 2011
Revised November 2012.
alan4alan at googlemail com
More of my pages related to SDR
All aspects of the Softrock SDR http://homepages.wightcable.net/~g4zfq/Si570.htm