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Juma TRX2A Info Needed Before Purchasing

PostPosted: 13 Dec 2015 22:04
by KF2YD
Hello All,

I am very interested in purchasing a Juma TRX2A all band CW, SSB transceiver and have some questions about the rig.

My main modes that I enjoy operating are CW and SSB but I also occasionally listen to some of the AM shortwave broadcast stations.


1) Some hams state that the RX sensitivity startes to drop off from 20M to 10M, is this true ?

2) Also some hams state that it has quit a few birdies across the HF spectrum, is this true ?


Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 16 Dec 2015 13:33
by 5B4AIY
Hi, Frank,

Let me see if I can answer your questions regarding the Juma TRX-2. As you are aware, this is a direct conversion SSB/CW transceiver using a Tayloe sampling mixer and a polyphase network for both the transmitter and receiver in order to achieve true single-signal SSB/CW performance. It uses an Analog Devices AD-9851 Direct Digital Synthesiser controlled by a Microchip dsPIC30F6014A 16-bit microprocessor for the local oscillator. The microprocessor is also used to control the functions of the transceiver as well as the LCD, but it does not sample the audio, and therefore the transceiver is not a Software Defined Radio.

AM Operation
The transceiver is not equipped with any form of demodulator that will optimally demodulate an Amplitude Modulated signal. Nevertheless, as with any SSB receiver, it is capable of demodulating AM, but with some caveats. This applies to any SSB receiver used for AM, not just the Juma. The first thing is that the AM carrier will beat with the local oscillator producing a beat note. Even if the local oscillator is exactly on the same frequency as the incoming carrier, there will be cyclic phase errors, leading to phase distortion. This is not particularly troublesome for speech signals, but unfortunately musical information is much less tolerant. There are a number of samples of the transceiver demodulating short-wave broadcast band AM signals on the Juma website, and I suggest you listen to them to gain an idea of how well it performs in this mode. I emphasise, this is NOT a fault of the Juma; rather it is because you are using a non-optimum method of detection. If you are seriously interested in short-wave AM broadcasts, then I would suggest you use one of a number of inexpensive Software Defined Radio modules currently available, such as the SDRplay and the FunCube Pro+ and then demodulate these signals using one of the enhanced detection modes available with the software such as SDR Sharp, or HDSDR.

Receiver Sensitivity

In order to usefully discuss this aspect, we have to first define what we mean by 'sensitivity'. There exists a de facto standard used by most Amateur Radio manufacturers, and this is to define the Minimum Detectable Signal (MDS) as that signal at the antenna input which gives rise to a 10dB Signal + Noise to Noise ratio (10dB S+N/N) in a defined bandwidth. Typically, the bandwidths used being 2.4kHz for SSB, and 500Hz for CW.

If I may digress slightly, let's discuss what this actually means. The theoretical noise floor is set by Boltzmann's Constant, which in SI units is: 1.381E-23 Joules/Degrees Kelvin. Now an energy of 1 Joule expended in 1 second is 1 Watt, thus it is also true to say that Boltzmann's Constant is equivalent to 1.381E-23 Watts-Hertz/K or, -198.6dBm-Hz/K. Usually we consider that the receiver is operating at room temperature, and for convenience we say that this is equivalent to 300K, (27C) so we would have to add 10*log(300) or 24.8dB to this figure to get the noise power in 1Hz at room temperature. If we are using a bandwidth of 2.4kHz, then we also have to add 10*log(2400) or 33.8dB to get the noise power in the SSB bandwidth, and altogether this amounts to -198.6 + 24.8 + 33.8 = -140dBm. This would be the noise floor of a perfect receiver. There is no such thing, and so we would also have to add the noise figure of the receiver to obtain the actual noise floor referred to the input. Most HF receivers have a noise figure of between 10dB and 20dB, so if we assume 15dB for the sake of argument, then the actual noise floor would be -125dBm. In order for there to be a 10dB S+N/N ratio, the input signal would need to be 10dB higher than this, or -115dBm. This corresponds to an antenna input voltage into 50 ohms of just 0.4uV. Equally, for a noise figure of 10dB, the equivalent input voltage for the same signal to noise ratio would be 0.2uV, and for a noise figure of 20dB, it would be 0.7uV.

Before considering the actual Juma performance, it is important to realise that for frequencies below about 15MHz for any reasonable antenna, atmospheric noise would dominate, and thus actual MDS figures are not particularly significant. Only for frequencies above 15MHz does the MDS assume some importance. By 'any reasonable antenna' I mean any antenna into which you would consider transmitting; only for very short whip antennas such as used for portable SW receivers would the MDS figure be important.

I measured the performance of my Juma TRX-2, equipped with the Spur Elimination module (more on this later) which I built some four years ago, and the results obtained are:

BAND SSB (2.4kHz) CW (500Hz)
I/P Power (dBm) I/P Voltage (uV) I/P Power (dBm) I/P Voltage (uV)
160 -110 dBm 0.7 uV -112 dBm 0.6 uV
80 -110 dBm 0.7 uV -113 dBm 0.5 uV
40 -111 dBm 0.6 uV -113 dBm 0.5 uV
30 -113 dBm 0.5 uV -113 dBm 0.5 uV
20 -121 dBm 0.2 uV -128 dBm 0.1 uV
17 -113 dBm 0.5 uV -118 dBm 0.3 uV
15 -120 dBm 0.2 uV -118 dBm 0.3 uV
12 -107 dBm 1.0 uV -109 dBm 0.8 uV
10 -110 dBm 0.7 uV -112 dBm 0.6 uV

As you can see, except for some slight loss of sensitivity on 12m, which might well be the input filter, the sensitivity is fairly uniform across the whole of the frequency range. Even on 12m the sensitivity is perfectly acceptable.

Compare this with any modern transceiver, and you will find that it is very similar. For example, the Yaesu FT-450D is specified as having a MDS in the SSB mode using a 2.4kHz bandwidth of 0.25uV, or -119dBm. The Icom IC-756 Pro-III manages 0.16uV or -123dBm. Compare that with my JRC NRD-525 communications receiver which is 0.5uV, or -113dBm.

Spurious Responses (Birdies)
When we allowed high-speed digital electronics, microprocessors, and digital frequency synthesisers into our communications equipment, we opened the door to a multitude of potential spurious signals. It is a fact of life that almost any modern transceiver will have some spurious responses somewhere in its frequency range. The Juma TRX-2 is no exception, and it would be foolish to state otherwise, there are spurious signals that are audible. Most of these originate within the direct digital synthesiser. The AD-9851 chip is specified as having a spurious-free dynamic range of about 73dB, but the TRX-2 has a dynamic range far greater than that, so it is inevitable that as you tune across its frequency range you will encounter various spurii. These take the form of one of three different types. The first appears as a 'normal' carrier, and tunes at a normal rate. The second type sounds like noise and tunes vary quickly. The third is also noise-like but tunes even faster. The first type is fortunately fairly rare, and is usually of such a level that unless you are testing the receiver using a well-matched and screened dummy load is virtually inaudible when connected to a reasonable antenna as atmospheric noise overrides it. The second and third types vary in level, but are usually not very bothersome because they tend to disappear with only a single step of the VFO knob. That is, they are very high-order spurious products and a shift of the VFO by sometimes as little as +/-1Hz or +/-10Hz completely eliminates them. These are caused by A-D conversion and phase truncation errors in the DDS chip itself. Unfortunately direct-conversion transceivers seem to be particularly prone to these artifacts. It is certainly true that an unmodified transceiver tends to be badly affected, especially as the frequency is increased and thus the local oscillator frequency now becomes a significant fraction of the master reference frequency. I discussed this with the designers some years ago, and they investigated and found that with improved isolation between the front end of the receiver and the input these spurii could be significantly reduced. I would thus strongly recommend that anyone contemplating building this kit purchase the Spur Elimination module and install this when building the transceiver.

I am attaching two PDFs to this post that summarise all of this. I hope it has answered your questions. If I can be of any further assistance, please let me know.

Adrian Ryan, 5B4AIY

Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 16 Dec 2015 17:22
by OH7SV

Thank you very much for your comprehensive answer. I have only two minor additions to your answer.

1. The Spurious Elimination Option (SE-option) is currently included in TRX2A kit package.
2. I am personally using only JUMA TRX2A and I have never lost a DX due to the TRX2A RX performance.

See also the reviews in eHam

Frank, feel free to purchase a TRX2A. I can assure that you will be a happy Juma TRX2A owner.

73 Matti OH7SV

Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 18 Dec 2015 21:55
by KF2YD
Hello Matti & Adrian,

I really appreciate your in depth and detailed explanation about the design and performance of the Juma TRX2A transceiver.

I was wondering if you had any plans in the future to design a new improved "Digital Synthesizer" for the TRX2A that will have a much better (SFD) Spurious-Free Dynamic Range ?

Frank - KF2YD

Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 19 Dec 2015 18:59
by F6DEX

I do have a TRX2A (without the Spur option because installation is critical) and yes, there is birdies but they are not annoying because they disappear very quickly. Now the new main board of the TRX2A includes the Spur reduction.
Sensitivity is also good enough on 10m (I also have a K3), no problem in real situations.
The only issue in my opinion is the CW bandwith of the roofing filter which is generally too wide ~800Hz (at least in critical situations).
Adrian 5B4AIY has made great improvements in the firmware to make the CW mode running about like an other rig (without QSK).

Best 73, Laurent F6DEX

Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 20 Dec 2015 17:33
by OH7SV

Thanks for your comments.

Here are some preliminary good news. We have just completed the design and test of a new enhanced JUMA roofing filter FL2 option for TRX2. The standard roofing filter is using 3rd/5th order topology and the new enhanced filter option is using a carefully designed 9th/12th order topology. The filter slopes are much steeper and the performance looks very good. It is very easy to install, just unplug the standard filter and replace it by plugging the FL2 option to the same pin header connector. No adjustment is needed and it is compatible with the official Juma firmware and third party firmware.

Kari OH7FVG is currently preparing the FL2 filter kit and it will be available soon. Watch JUMA Shop ... oductId=26

73 Matti OH7SV

JUMA Roofing Filter FL2

Re: Juma TRX2A Info Needed Before Purchasing

PostPosted: 20 Dec 2015 17:38
by F6DEX

EXCELLENT !... I studied the problem myself because I thought that a slight change in the value of the components of the active fillter could reduce the bandwith. This is not an issue of the rig just a design choice. I see on you image that the change is more complex.

I hope it is usable with the excellent firmware of Adrian...

Many thanks, Laurent F6DEX