Monday, 5 August 2019

IC9700 again

I have been busy remeasuring the reciprocal mixing dynamic range (RMDR) of the IC9700 receiver on 2m and 70cm.
Recent measurements have shown that the IC9700 has an RMDR of well over 90dB on both 2m and 70cm at close spacing (~10kHz) and even better further out.
This is a key measurement of a receiver’s dynamic range and is critically dependent on the local oscillator. Conventional superhet architecture receivers can be particularly bad. The fixed LO (clock) of an SDR receiver makes the design of ‘quiet’ local oscillators much easier than with an LOnthat must be tuneable as in VFO.
Older superhet receivers have typically given a RMDR in the 70dB range. More recent designs have improved this into the 90s and even as high as 115dB. However, this performance comes at a price. The SDR direct sampling  receiver can achieve high levels of RMDR at a much more acceptable price.
I will be presenting my measurements and those of a number of well-known experts at the RSGB Convention in October.

Sam

Sunday, 21 July 2019

4m noise

4m has always been  noisy here.
Using the arrangment in the previous blog, but on my IC7300, i was able to measure the noise level around my antenna.
In the worst direction (towards the house) the noise rose 18dB over the quietest direction. Not good!
I need to investigate what is producing that noise increase.
Now to repeat the measurement on the other  bands  covered.

Sam

Measuring noise using the IC9700

No, I didn’t get on the moon on 6cm for the Apollo EME commemorative event. Chesty cold knocked me for six. Just recovering. Courtesy of our  vistors from Australia........l

It did allow me time to consider how I could make continuum noise measurements using the IC9700 (on 23cm) when it doesn’t have an IF output in the usual sense. One thought was to try and use the 12kHz IQ IF output, but I failed to find any information on this or how it could be used with existing SDR software. I know it can be done.
I had played with using my SDR-IQ at audio (it goes down to 500Hz) to measure noise power in a defined bandwidth using Spectravue running on the SDR-IQ. This worked well, but I was not happy with the amount of amplitude ‘jitter’ on the trace.
I managed to find  a working HP3400 true RMS voltmeter at Friedrishshafen, to do the same measurement with a ‘proper voltmeter’!. Again flicker, because of the nature of noise and the restricted bandwidth, made this unsatisfactory.
Finally, I set up the IC9700 as a soundcard input to Spectravue (no SDR-IQ) and selected continuum mode. With a moderate amount of FFT and smoothing the trace is as stable as it was using the SDR-IQ on the K3 8.215MHz IF output. Not only, but the results agree very well with those previous  IF measurements.
Now I CAN use the IC9700 for EME noise measurements!

Sam

Thursday, 4 July 2019

Apollo commemoration - 6cm EME

Some of you may be aware Dean, VK6DSL, is working to get the 28 metre diameter dish at Gnangara, Western Australia, operational on 5.7 GHz EME to commemorate the Apollo 11 moon landings. Brian, G4NNS has been working to get Goonhilly GHY6 on air as GB6GHY as part of the celebration.
I thought it might be nice to try and get my 6cm EME system back on air to try and work them both. I haven’t used the 6cm system for about a year. As the local oscillator was not stabilised and the DB6NT MKU57 G2 transverter was located close to the dual (combined) power amplifier, if suffered badly from frequency drift due to heat. I never quite got round to modifying it for an external high stability LO source.

I viewed the new MKU57 G4 on the Kuhne Electronics stand at Friedrichshafen, last month, and decided I would buy one, especially as there was a small discount. 
To help things along I had been asked if I was prepared to sell my G2 and even my old G1. Of course!

The G4 has an external 10MHz reference input as well as a good internal TCXO. This should give me the frequency stability that I require. Doppler is bad enough to track without having to adjust for thermal drift as well.

I have decided fo locate the G4 away from the PA and LNA. That probably means at the back of the dish. That has several advantages.removing the transverter from that heat source and reducing the weight of the feedpoint equipment because I can also remove the control box and sequencer from the feedpoint.

Two low loss coaxial cables will connect the feedpoint PA/LNA/feedhorn. Both the high gain of the PA and the reasonable gain of the LNA  (a DB6NT LNA  - I don’t make one for 6cm) will enable me to run this arrangement with negligible receive system sensitivity loss and no transmit power loss.
One of the shortcomings of my previous arrangement was that the SSPA relied on simple conduction cooling to the aluminium frame at the feedpoint. Whilst this never led to any failures of the PA, a better cooling arrangement was a definite requirement.
With the transverter removed from the RF feedpoint frame I am now able to fit a 12V fan to blow air between the two PA modules that comprise the dual balanced amplifier. I found in test that I could increase airflow by cascading two identical 12v fans, one blowing into the other. In this arrangement the two fans tend to synchronise and definitely increase airflow. Whether it is twice a single fan is still unknown.....but it’s more than a single fan.

Sam




Thursday, 13 June 2019

IC9700 v1.11 firmware upgrade. Updated from previous blog

First a bit of advertising.
My review of the IC9700 is in the current issue of Practical Wireless, out today, for those of us without a subscription.

Having now upgraded to V1.11 firmware, in order to improve the reference stability, here are my observations.

By now the IC9700 reference oscillator stability is a well-known problem. In fact, for most users of CW, SSB and FM, the stability is a non-issue. It is only with the narrow digital modes that the stability problem shows. It seems to be associated with poor thermal control around the 49.xMHz reference TCXO.

The cooling fan comes on on transmit only. It does not run continuously. Not only that, but it only comes on when required on transmit. When it does come on it causes a small, but noticeable drift in frequency. Without the fan coming on, drift is much less noticeable.
On 2m it is hardly noticeable, even on FT8. It is more noticeable on 70cm and definitely noticeable on 23cm, although not really a big problem using JT65C on 23cm, compared to many other rigs.

Several ‘fixes’ have been proposed, including causing the fan to run continously, albeit at half speed; full reference locking using an add-on board and the possibility of a firmware upgrade to implement a locking system in the internal FPGA.

As designed, the external reference 10MHz input is little better than a modern version of a crystal calibrator. A facility is provided to automate the calibration of the internal reference against the external reference input. Once calibrated in this way, the internal reference is once again free to ‘drift’,  controlled only by the TCXO function.

Version 1.06 firmware improved on the correction algorithm and did make a big difference. The IC9700 is able to achieve a frequency accuracy only dreamed of in rigs like the TS2000x without frequency locking mods.

With firmware v1.10 and now V1.11the reference is periodically corrected, once invoked, and now works much like the system used in, e.g. the K3.
However, the period is possibly a little long as there is still a noticeable shortbterm drift under some conditions. I can see some further work, to refine the process, may yet be forthcoming from ICOM.

So, how well does V1.11 work to stabilise the reference oscillator?
On 2m FT8 I could see no change in frequency between transmit and receive, fan operating or not. Well, maybe a Hz!
FT8 on 70cm barely showed showed any change. Certainly not enough to cause any problems.
As FT8 is not recommended on 23cm, I didn’t bother to check for change with these short, 15 second, transmit/receive periods. However, JT65 uses a 47second transmit period followed by a 1minute, 13 second receive period. This puts a much bigger strain on the thermal characteristics of the reference oscillator and drift, even with the wider spaced tones of JT65C, was previous noticed with drift exceeding 20Hz when coming back to receive after the 47 second transmit period. This is important as any difference in the tone frequency from the expected frequency leads to a small but definite degradation in sensitivity. Keeping the tones where they should be gives the highest sensitivity.

So does V1.11 improve the situation at 23cm.
Basically, yes. 
To test the drift I set up one of my signal generators to produce a very weak reference carrier signal about 2.4kHz above the dial frequency I had chosen for my JT65C tests (into a summy load). This carrier produced a nice, straight, line on Spectran. The span on Spectran was set to show just a few hundred Hz either side of the tone. A drift of 10Hz was quite noticeable at this setting.
WSJT-X was then set to give repeated CQ calls of 47 secs on followed by the receive 1minute 13 second od receive, during which the 2.4kHz tone could be seen and measured. This was done at maximum 23cm output power (100% or 10W). 
As the signal generator was GPS displine-locked the frequency of the test carrier was very stable.
On test the tone frequency was always within a few Hz of 2.4KHz when returning to receive. It would then wander up to a peak of about 10Hz HF (2.410kHz) over the first few seconds, returning close to 2.40kHz reasonably quickly. Sometimes overshooting to a few Hz below 2.40kHz.
This happened each tx/rx cycle (over) although the peak excursion didn’t always reach 10Hz and averaged about 5-8Hz.

Although not yet perfect, this is a big improvement over the v1.05, v1.06 and v1.10 firmware.

I would expect a further improvement with a refined version of the firmware.

I am quite happy with this result. It would be interesting to do this level of testing with a few of the other transceivers covering 2,70 and 23cm!

I know from experience that the TS2000x drifted about 50Hz between transmit and receive on 23cm JT65C, before doing the fan-always-on mod.
This suggests that it might be worth still doing this mod to the IC9700 to keep the fan turning all the time, as when run at low power on 23cm (1.5W) the fan rarely comes on and drift was noticeably less in these tests.

More testing to do!

Sam






Thursday, 6 June 2019

QO100 portable receive system

It is our local club(s) rally on Sunday. ESWR has been running for many years and is the product of several local  clubs.
I seem to have been given the job of organising the UKuG guest interest group stand. I’m a bit surprised as I am no longer the Group Chairman and not even on the Committee this year. However..........
I have asked Jason, G7OCD, to bring his portable QO100 uplink system that he has demonstrated at several local clubs. To save effort his demos have usually used the web SDR, located at Goonhilly, for receive.
I felt it more appropriate that we demonstrate a ‘real’ portable downlink at ESWR. To that end I have been  busy.

Earlier this year I bought a portable ‘caravan’ satellite dish and stand. The dish is an offset 45cm steel unit and fits to the tripod stand. Instead of the original low quality, low cost, LNB, I decided to use a Gooby LNB (£5.60 each on Amazon). These are 25MHz crystal controlled PLL LNBs.
Obviously, the crystal in the PLL is of insufficient quality to provide good stability when mounted out in the open on the dish.
I decided not to use an external 25MHz reference, like I use in my home station satellite system. Instead I decided to try the beacon locking system now incorpoated in several pieces of software. I decided to use SDR Console v3.0.9
My initial attempts to run this version of SDR Console on my now elderly Sony Vaio laptop resulted in the program freezing, so I was starting to think about running unlocked after all. Sunday is getting close.

This evening I decided to try the program one last time and to my surprise and delight, it worked first time. I still don’t know what I was doing wrong previously.

With SDR Console I select ‘geostationary beacon’, then follow the instructions and the software locks on to the PSK beacon and stays there when I tune away to listen to stations. It is not foolproof and does still show some drift, although none of any consquence in the time I was listening. Some of the drift may also have been due to the reference in the RSP2Pro SDR I was using at 739MHz, also contributing to drift. Inam goingbto check that my locking the RSR2pro usingbits external reference input.
.
My laptop drives the RSP2Pro and supplies its voltage via the USB socket. The Gooby is powered from a portable 12v 12000mA/h lithium pack (Lidl) and seems quite happy. I use a protected-against-short bias tee to supply the LNB over a single coax.

Now, I hope, with Jason supplying the 2.4GHz uplink, we will have a working 10GHz downlink to complement it to show rally visitors all about QO100.
The gazebo and table and chairs are being provided by other local  club members, so I don’t need to worry about them!

73 de Sam



Sunday, 19 May 2019

G3JVL

I was sorry to receive the news that Mike, G3JVL, has passed away.
I first met Mike back in the 1970s at the Crawley Court, Winchester, Microwave Round tables.
Subsequently, I joined the RSGB Microwave Committe back in the 1980s and Mike was afellow member.
We had a lot of dialogue during those years, Mike made me a 2.3GHz quad loop yagi that I used for many years and then in the 1990s I visited with him and saw his home station. From that I was loaned a small (2.5W) TWTA that I used in my home 3cm station. I had never seen a TWTA with only three (or was it 4?) connections previously. Mike showed me how to use it and I had many long distance QSOs on 3cm,  not only with Mike, but also with many of the other 10GHz narrowband home and portable stations that were around during the 1990s.
I later met Mike, again, when we were both attending the Plano, Texas, Microwave update. I think that was around 2000. Mike was visiting with his daughter who, I recall, lived local in the Plano area of Texas. 
I don’t think I ever saw Mike again. I am sure he continued active to be active on the bands,  but our paths never crossed again. Ioften wondered what he was up to.
I guess I will never know,  now.

Vale Mike.