X-Phase — More theory

The advert is from Practical Wireless February 1987.

In my last blog I tried to explain how a QRM Eliminator works. Here’s some more information.

Suppose the signal you are trying to hear is a sine wave (shown in blue). The signal as received (shown in red) will have some noise added as it travels to you.Orig noise

The QRM eliminator allows you to pick up the noise with your noise aerial and phase shift by 180º. As shown in this chart.

Noise

If you add the blue and red signal together you’d get a zero signal. So if you mix the inverted noise with the noisy signal as received you’ll recover the original signal.

Recovered

Of course reality is different and the recovered signal won’t be as clean as that. Also if you don’t match the noise and main signal amplitudes properly, you’d then get something like this.

Recovered loud

These charts were made with MATLAB using this script.

X-Phase QRM Eliminator

I bought an X-Phase QRM eliminator a while back, tried it out with a receiver and was quite impressed with its performance. It’s only recently that I’ve connected it to a transceiver because without care it is easy to damage the unit when transmitting.

QRM eliminators have been around for many years. I was recently looking at an old Practical Wireless from 1989 and S.E.M. were selling one then in the adverts at the back of the magazine. (And, yes, we used dots in a.b.b.r.e.v.i.a.t.i.o.n.s. back then). If you were to be picky you might say it should be called a QRN eliminator, but it isn’t. I quite like the idea of an actual QRM eliminator though I’m not sure how you could implement it. 

Scan 9 May 2020 at 14 34

A QRM eliminator works like this: signals from the main aerial are mixed with signals from a noise aerial. The signals from the noise aerial can be shifted in phase. The idea being that you mix the main signal with the noise signal 180º out of phase. If the signals are the same you’ll just get a zero signal. But if the main signal comprises a good signal and some noise signals and the noise signal is predominantly the noise signals, you’ll end up with just the good signal. Of course, to make this work you need to be able to make the noise signals from the main and noise aerials be the same amplitude so the QRM eliminator has controls to adjust the gain of each. As you want the signals to be 180º out of phase there is also a control to adjust the phase.

Front of QRM Eliminator

The three blue knobs in the photo are these controls.

There are several QRM eliminators on the market. I got mine from Poland on eBay from the seller urbania2. The unit is solidly built in a neat aluminium box with pleasant to use control knobs and strong connexions on the back. 

The instructions are in a quaint mixture of Polish and English but I found them understandable enough as a circuit diagram is included.

I have done some quick tests on 20m with the unit and it seems to be able to reduce the background noise by about 3 S-points as shown on my TS590S transceiver. I also made some measurements using received FT8 signals. This showed an increase of about 4dB in the signal strength of CQ signals as reported by JTDX over 15 minutes with the QRM eliminator being on each even minute and off each odd minute. None of this testing was particularly scientific though. I was using a 4m length of loudspeaker wire as the noise aerial, just lying on the floor of the shack.

Eliminator connexions to TS590S

I mentioned that it’s easy to damage the unit when transmitting. It has three wires. Red and black are for the DC supply, and the yellow wire is for the PTT. When grounded the eliminator passes the main signal straight through avoiding the damage.

This Is how I connected my TS590S. The EXT-AT connector on the TS590S provides a nominal 13.8V DC. So I used pins 1 and 6 to power the Eliminator. I got the EXT-AT plug from an eBay supplier asia_uk.

TS590S EXT AT Connection

The remote connector on the TS590S isn’t particularly well documented, but connecting the yellow wire from the Eliminator to pin 4 works, but only if menu 53 on the TS590S is set to 2. Pin 2 on the remote connector, the common terminal needs to be grounded so I connected it to pin 3 on the EXT-AT seeing it was spare. I got the required 7-pin DIN plug from RS Components.

TS590S Remote Pinouts

SignaLink Jumpers for FT290R

Here’s how to set the jumpers in a SignaLink USB when connecting a Yaesu FT290R.

FT290R

I found the jumper instructions on the Tigertronics website just a little too general, so this may save some time.Signalink jumpers for FT290R IMG 1043

The SignaLink works fine with PocketPacket on a Mac mini. ‘Use Vox for PTT’ was set in the PocketPacket Audio Modem preferences. The SignaLink delay knob was turned fully anticlockwise.

Using this setup I could receive and decode signals from a local packet test GM7RYR-10. I transmitted to the ISS packet digipeater but didn’t see any of my packets digipeated. However, I received my packets locally on my Yaesu FT60 and decoded them on a Raspberry Pi via Direwolf by WB2OSZ and Xastir.

PTT for FT290R

I bought an FT290R (thanks Bob!) a few months back and have finally got around to trying it with packet. The PTT circuit I used for the FT60 Raspberry Pi 3B+ works fine so all it needed was to connect up a plug for the front socket as shown here.

PTT for FT290R

I tested it with Direwolf and Xastir and it seems to work fine. Here’s an audio clip FT290R Packet Audio.flac of a packet being sent from the FT290R. I recorded it using Audacity from my Alinco DJ-C6.

I had hoped that the extra power (25W) from the FT290R would allow the ISS to hear my packets but I’ve had no joy in the couple of passes I’ve tried so far. I can hear packets fine, but the ISS doesn’t digipeat the ones I’ve sent. I changed the APRS path to just ‘ARISS’ having read this aprs.fi blog but that didn’t help either. Perhaps the ISS needs to be at a higher elevation. Or perhaps my rather ramshackle Cebik Moxon aerial needs tweaked.

Cebik Moxons, SDRplay and satellites

There was a pass of the AO-91 satellite over my location today and I listened to the ham radio operators operating through it.

I used my home-built Cebik Moxon aerials which are located in my attic and the nice SDRplay RSP2.Cebik Moxons in attic

Here’s a screenshot of SDRuno displaying the AO-91 signals for those of you who don’t think the Doppler effect is real.

Screen Shot Doppler Effect

You can easily see the received signal changing frequency as the satellite hurtles past.

This is what it sounds like. 

AO-92 12.09 Wednesday, May 8, 2019 20190508 1209.m4a

It was recorded using Audio Hijack Pro from a Microsoft Remote Desktop session of SDRuno on a Dell XP workstation. Apologies for the over-driven audio — I was concentrating on receiving rather than recording.

SDRplay Macs and Linux

The Good News

I bought an SDRplay RSP2 recently and have been enjoying using it a lot. The RSP2 has three antenna connections and covers from 1kHz to 2GHz. It is amazingly good value. SDRplay provide a nice receiver application called SDRuno. The SDRplay website has links to reviews of the RSPs and they must be pleased with them.RSP2

The Bad News

My shack is full of computers accumulated over the years. However, as far as Microsoft Windows machines are concerned I only have an ageing Dell Precision 380 running Windows XP and a VirtualBox VM on a Mac Mini running Windows 10. My W10 VM isn’t fast enough for SDRuno and the audio stutters. The XP machine is usable as long as the sample rate is kept low and you decimate a lot.

On Macs and Linux computers SDRplay only provide an API/HW Driver, although they do provide a full image to boot a Raspberry Pi from. The software for Mac and Linux is CubicSDR which just about does the job but does not have all the features of SDRuno. I prefer GQRX  and have managed to get it working on macOS but the Hi-Z antenna connexion only works intermittently. It’s all quite unsatisfactory. It’s frustrating to have such good hardware spoilt by the lack of easy-to-install software. You get sucked into handling a morass of libraries with differing versions and it’s time-consuming if not impossible to find out which versions you need to use.

The Raspberry Pi image works well as long as you connect an HDMI display. I prefer to run my Raspberry Pi through Microsoft Remote Desktop so I don’t have multiple monitors, keyboards and mice on my desk. But with Microsoft Remote Desktop CubicSDR does not display well at all.

I only hope that SDRplay comes out with SDRuno on other platforms other than Windows. 

In Other News…

So I’m currently using the Dell XP computer with SDRuno run through Microsoft Remote Desktop as the best solution for me, even though it isn’t supported. Don’t worry, my insecure Dell XP is only connected to my LAN and is not connected to the Internet.

The blue arrow points to the low sample rate and high decimation needed on the Dell XP.

SDRuno Screen Shot

But it works well enough, it just doesn’t use the capabilities of the RSP2.

SDRuno Maiin Panel Screen Shot

Cebik Moxons in the Attic

I now have both 2m and 70cm Cebik Moxons in the attic separated using a HA8LFK diplexer. After trying out the 70cm aerial it is clear that it needs a preamp to pick up any signals. Partly this is due to the poor coax I’m using and I should replace it with something better. So I’ve added an M-100 preamp powered from a shack PSU through a pair of bias-tees.

I tried bypass the M-100 with a relay but I haven’t been able to source a relay that works well at UHF. Usually the non-connected path is only 20 or 30 dB down on the connected path. This is not safe enough to protect the preamp, so that’s another reason for giving up transmitting for now. I’m pretty sure the aerials will not be good enough for decent transmissions and I would end up being the recipient of pleas from an OM: “I got the Mike, Zero and Yankee, your callsign again again please…”.

The setup looks like this scribble from my lab book:

Cebik Moxons Diagram

Unfortunately the M-100 draws 55mA which is 5mA too high for the built-in bias-tee in the SDRplay RSP2. The RSP2 is my receiver of choice at the moment. More on that later.

SDRplay Safety

 

I bought an SDRplay RSP2 SDR recently. I had been looking for a multimode VHF/UHF transceiver but couldn’t find one within my budget. The transceivers that get listed on eBay go for silly prices and I’m not sure I want to pay the asking price for a new model. 

So I’ve decided to concentrate on receiving signals from satellites rather than transmitting, hence the purchase of the RSP2.

One of the specs of the RSP2 is the maximum signal it will tolerate at its aerial inputs. It says 0dBm which by my calculations says the voltage at the input must be under 0.22 Vrms to be safe. This sent alarm bells ringing as my Cebik Moxon aerials share the same attic as my small transmit loop aerial.

So I made some measurements.

I measured the voltage seen in the shack from the Cebik Moxons when transmitting 10W to the magnetic loop. The measurements were on my HP 54615B scope with a 50Ω termination.

No RF 800µV
TS590S tune (10W) 191mV at 14.2 MHz
TS590S tune (10W) 288mV at 7.07 MHz

So my RSP2 is unsafe in its current location and configuration. I must not transmit using my HF rig if I have the RSP2 connected to my Cebik Moxon aerials.

My first thought was to put a high pass filter on the Cebik Moxons to block 6m and below. I’m sure this would work but there would be some insertion loss.

But I have an Moonraker M-100 masthead preamp which has a 24-2300 MHz filter. This wouldn’t block 10m and 6m, but I don’t transmit on those bands using the small transmit loop and if I did the radiated power would be quite small as the impedance mismatch would be high.

So I made some more measurements with the M-100 in the shack. I know these won’t give the same effective gain at the receiver as measurements made with the M-100 in the attic, but it’s close enough for a start. The dial on the M-100 is laughable as the gain you get at each position bears only a passing relationship to the actual gain. The most you can say is the more clockwise the dial the higher the gain. Hence the ‘on dial’ below.

• Zero gain (on dial)
No RF 18mV
10W 14.2MHz 32mV
10W 7.07MHz 39mV

• gain +10 (on dial)
No RF 18mV
10W 14.2MHz 25mV
10W 7.07MHz 53mV

• gain +20 (on dial)
No RF 19mV
10W 14.2MHz 559mV
10W 7.07MHz fuzzy on the scope so I assume the signal is distorted somehow

• gain at 12 o’clock
No RF 18mV
10W 14.2MHz 50mV
10W 7.07MHz 16mV

This looks promising as the preamp gain should be set to just retrieve the loss from the cable and this should end up being 12 o’clock or lower.

The next step is to install the M-100 in the attic and re-measure. I’ll probably use a bias-tee arrangement so I can power it from the shack.

 

 

 

CHIRP Programming

I use a Yaesu FT-60 for my satellite radio. It’s a little onerous to manually  program satellites into it so I use the CHIRP application to do the programming.

I can’t use my FT-60 programming cable directly on macOS (my native OS) because it has a Prolific chipset for USB/serial operation and its driver clashes with the FTDI chipset which is in other USB/serial devices I use.

I first used a Windows 10 virtual machine (VM) under VirtualBox to run CHIRP. This worked fine. But I only use Windows 10 sporadically and when I start Windows 10 it nearly always wants to reboot to do an update. This is tedious and slows things down a lot. I spent more time waiting for Windows to update than running CHIRP.

So I’ve put a Ubuntu VM onto VirtualBox and now run CHIRP quite happily from there. Hopefully Ubuntu won’t insist on upgrading quite so often as Windows 10 does.