Thursday, 19 November 2015

Using Time Domain Reflectometry to align filters

Time Domain

Frequency Domain
Here is my first attempt at using TDR to align a 437 Mhz BPF filter.
The technique is described in Agilent AN 1287-8
The dips in the Time domain represent the resonators in the filter, S11 from one
end of the filter and S22 from the other. TDR allows you to see
the results of adjusting each resonator fairly independently of the others.
The aim is to increase the dips as much as you can. The peaks are related to
the inter resonator coupling. For a full explanation you need to read the Appnote.

As you can see from the frequency domain plot my efforts were not perfect
but I suspect practice will make perfect. The tune elements were 6 mm brass
studding and these were not the easiest thing to adjust. After I had done the
measurement I got a warning saying the calibration had been aborted. I had been
at it for about 30 mins and was getting cold, there is no heating in my workshop.
I will however try again. The filter is adequate for my 70 cms DATV station.

Sunday, 8 November 2015

First steps with amplifier distortion investigation

Power Sweep
This is my first tentative steps at looking at distortion in PAs using the HP8753E.
Tentative really is the most appropriate word as one slight error and the VNA could
be compromised. The maximum input power the receivers can accept before
damage is only 20 dBm. It is important to have a lot of attenuators in place.

The above graph is that of a RA60H1317M operating at 146.5 MHz.
The output starts at around 12 watts  and ends around 70 watts.

The top trace is the phase difference across the power sweep and the bottom trace
the gain. I didn't calibrate the gain to take into account the 30 dB attenuator or the
effects of the directional coupler so it is a relative value rather than an absolute one.

The drive was also too high, next time I attempt this I will use the 70 dB stepped
attenuator in the test box and I will also properly calibrate for the attenuators
and couplers.

To measure the output I used a 20 dB directional coupler and a 30 dB Narda
attenuator which reduces the 70 watts (48 dBm) down to about - 2 dBm.
The maximum measurement input level of the VNA's receivers is 0 dBm, above
that they start to go into compression.

Saturday, 31 October 2015

Re-purposing my Linux program for the HP8753E VNA

Linux App for the HP8753E VNA

Just a quick update. I re-purposed the Linux program I was writing to use
with the HP8753E VNA I recently bought. Currently the App can download
scattering parameters from the VNA and display them on a custom widget.
It can also save those s-params to disk in .s2p format, read them back and display them.
All this is done using the GPIB interface and a Prologix GPIB to Ethernet adapter.

It is very slow, to download a complete set of s-parameters with 201 points
takes about 30 secs. I will add more facilities as and when I need them.

Thursday, 22 October 2015

VNA update

HP 8753E opt 011
A while ago I started to design a VNA (Vector Network Analyser) but the more
I looked into it, the bigger the job it became. I am currently trying to focus on
Zynq based developments so I decided to drop the VNA project.
The parts I have accumulated will instead be used for a signal generator.

So I bought a used HP8753E opt 06/011 (deleted test box) from the U.S.
I already owned the matching S-parameter test set. I chose not to get one
with an integrated test set as having an external test set means
you can replace it with a coupler for things like high power amplifier design.
In fact after graduating and before I went into Software Engineering that is
what I did for almost 2 years, designing broadband amplifiers.
I wasn't very good at it!

So far I have updated the firmware to the last version created by
HP/Agilent/Keysight and once the limited warranty expires on the unit
I will add a couple of extra software options (requires opening the
box to set it to update). The software keys are available from
someone on the hp_agilent yahoo forum I believe, alternatively
you can alter some address locations in memory to add the options.

I am still after a set of printed manuals for it (I have the electronic versions),
unfortunately while the manuals are inexpensive and plentiful in the U.S once
you add the postage they become quite expensive. So I shall wait until
I see some UK based ones.

So far I have managed to find that one of the leads I made up a couple of
years ago was defective and that the 5.7 Ghz Evanescent filter I made is
very lossy. The fault in the cable was to do with how the braid was
attached to the N connector, reassembling the cable in the proper way
made a huge difference to the return loss of the cable when terminated
in a 50 ohm load.

With around 100 dB dynamic range the results are considerably better than
the Rigol DSA 815 with tracking generator I was using before.

I have also managed to get screen grabs of the analyser using KE5FX GPIB
tool set but I have not managed to save S2P files yet.

Thursday, 15 October 2015

DATV-Express Windows Apps

DVB-S Server Application
SSB/FM Transmitter

Hopefully later this week the above two applications will be finally released.
As you can see they are slightly different than before, reflecting comments
from the initial batch of users.

The SSB radio has had the ability to use a mouse wheel and an option to
transmit an unmodulated carrier added. Also I have suppressed the
most significant digits on frequencies below 1 GHz.
I don't really know how far to take this development as it is really just a
bit of fun. Personally I plan to use it as an exciter for my 10 GHz
transverter and maybe to drive a PA on 13 cms for the up and coming
E'hailsat narrowband transponder.

The DVB-S server software has had a long standing issue with the FPGA
code corrected. A later release will also have an extra interpolater added
that improves aliases between 1 MS/s and 2 MS/s.

I am hoping to get back to my work on the Zynq platform now. In preparation
I have been reading Adam Taylor's "The microZed Chronicles - Using the
Zynq 101" on the Xilinx site. I even spent £10 and bought the first year of his
chronicles from Amazon.

Thursday, 17 September 2015

SSB using DATV-Express

DATV-Express TX SSB/FM Application

Just a quick preview of a small application I have been working on that turns
DATV-Express into an SSB/FM transmitter. Originally I used a couple of FFTs
to do the IQ generation, filtering and interpolation. Now inspired by
Dave, W9GR's article in QEX on CESSB (Controlled Envelope Single Sideband)
I use the Weaver method of SSB generation combined with  his clippers.
I currently interpolate the output signal by 16 to 128 KSamples per sec using
halfband filters before sending that to the DATV-Express board via USB2.
On the Express board the signal is further interpolated by a factor of 64 to reach
the final sample rate.

The high sample rates is required because DATV-Express has fixed 5 MHz LPFs
for anti-aliasing, after all it was designed for digital TV rather than narrowband SSB.

The RF level sets the output level of the DATV-Express board and the AF level
sets the input audio level from the PC's microphone.

The application still has some rough edges. I wrote it so I could have something to
drive the 10 GHz transverter I have built  which requires an IF of 1 GHz.
The transverter  uses a  9 GHz YIG based oscillator for its LO.
For receive any number of cheap SDRs could be used.

The processing overhead is relatively low, I am hoping it will be low enough to run
on the Pipo X8. Something I will try later today.

For those unfamiliar with DATV-Express, it is an exciter board that produces
around 10 dBm output which will transmit anywhere from about 65 MHz to
2.48 GHz. The board has been designed for the generation of digital TV on
Amateur Radio frequencies.