Going away notice

Hello,

I will be away for a week and will not be able to process any orders. Please if you need a kit or boards, place any orders before tomorrow lunch time GMT(Wed 12th July). After that i will disable the Order page for few days, sorry about that. I will also have limited ability to respond to support requests via e-mail, apologies in advance for that.

73, Chris

Correct position of D5/D6

Hello,

As a follow up on my previous post, here how to replace the D5 and D6. The white multi-line on the component is the cathode. On the PCB footprint, rounded corners on one side indicate the cathode position. As they say, a picture is better than 1000 words:

(click for larger view)

And while removing the old parts, please make sure you follow the common sense approach:

  1. Wet the place with lots of liquid flux
  2. Replace solder on the pads with good quality 60/40 lead solder
  3. Heat up with enough heat (320 deg C) and alternate between two pads. Use absolute minimum force. Or use solder braid to heat up both pads the same time
  4. Clean the pad from flux before soldering the replacement part

73!

Complete Kit v 0.6 Assembly Error

Hello,

Unfortunately there has been an assembly error on the v 0.6 complete kit. Wrong component has been installed for D5 and D6 on the RF board. Although same component is used for D1 as well, it is not critical to change it. But D5/D6 are part of the SWR/PWR bridge and it is important to install the correct component. Please check if your RF board looks like that:

And if yes, please drop me a mail with your postal address. I will send you the correct replacement parts, free of charge.

I am sorry for the trouble, it was my error causing wrong BOM file sent to the factory.

73, Chris

M0NKA

Complete kit v 0.6 is in stock

Hello,

After the long wait, the complete kit, v 0.6 is available. It is no different than the v 0.5 kit – two PCBs (UI and RF) with pre-installed SMD parts from factory, two plastic bags with post assembly components that you need to install and LCD. Again no speaker or final mosfets. Lots of information about the new revision on my Downloads page.

73!

Kits and PCBs out of stock

Hello,

Temporary the components kit is out of stock, due to shortage of v 0.6 PCBs. I will get some next week hopefully and can offer blank boards and the components kit again. Sorry about this everyone.

The only thing i have ready to ship is few v 0.5 RF blank boards, if anyone is interested.

73

Components kit v 0.6

Hello,

Have been a while. I have managed to prepare a limited number of the good, old components kits. But updated to PCB revision 0.6. Please note, this is not the Full Kit with pre-assembled SMD components from factory! It is just two blank PCBs(v 0.6) with four bags with pre-selected components – two bags with SMD parts and two bags for post assembly (after PCB cleaning). Also included is the LCD, but no MOSFETs or speaker as usual. If interested, please check the Order page.

About the 0.6 documentation – probably tomorrow. New schematics, BOM, packing lists, pdf with full description of changes, etc…

The full kits and blank boards v 0.6 would be available probably in the middle of March. Sorry for the long delay, i have been caught badly by the Chinese New Year this time.

73!

E-mails response delay and Full Kit Stock

Hello,

Apologies to everyone who sent an e-mail in the last few days and didn’t get a reply, unfortunately i caught some winter bug and was in bed, unable to move or do anything. Finally feel better to catch up with work stuff.

The main question in comments in e-mails is about the Full Kit stock status. I was not able to arrange the manufacture of the new batch before the Chinese New Year, so I really can’t tell when the Full Kit will be back in stock. My best estimate is March, but will keep everyone posted.

I am also preparing some components only kits (where you have to solder everything yourself, but parts come pre-selected). But still some of the orders didn’t come through, so after sorting everything, i will know i can offer those, i hope in two weeks time, but again not sure.

73

The mcHF pre-amp revisited

Few months ago I had interesting conversation with a colleague who got a kit from me and built it. He was using it on 27 Mhz and according to him, the breakthrough from the Local Oscillator (LO) could be heard two miles down the road via another receiver. This was interesting problem to investigate, i could have worked on a fix immediately as i have a Spectrum Analyzer, but it would have been foolish to ‘play’ with the RX mixer and pre-amp without having proper baseline measurements for sensitivity. And this was not possible as the Tracking generator of my Spectrum Analyzer is stuck to specific output value, i think the attenuator circuit is cooked, maybe the previous owner was careless and transmitted into it.

marc

So after some searching on Ebay, watching videos from Dave (EEVblog), i have narrowed it down – i desperately needed Marconi Aeroflex 2024 RF signal generator. I was lucky to get reconditioned unit in almost new condition, ex-MOD. It even had Option 4 installed – the oven oscillator! The most incredible thing about this signal generator is the built-in precise attenuator, with sub dB steps, that can go as low as -140 dBm. It has also great protection against accidental TX into it’s output port of upto 50W! And it can switch units on screen – either dBm or uV.

The Test Setup

I was ready to to test the sensitivity and LO leakage on the antenna port. I have chosen to test MDS (Minimal Discernible Signal) while using simply my headphones. This method is highly subjective, but it was ok for my comparative testing of improvements and mods. It is possible to do a fully scientific measurements, but this is beyond my current needs. I have decided to use my Audio Technica ATH-M50 studio headphones, plugged directly to the mcHF headphone jack.

headp

Despite the protection of RF signal generator, the first thing is to disable TX from the Advanced menu of the mcHF. Being extra careful never hurts! I am using v 0.5 RF + UI board for this test.

Sensitivity test

The results from the sensitivity test(MDS, 3.6kHz filter, RF gain at 50):

  • 80m, -118 dBm
  • 60m, -109 dBm
  • 40m, -118 dBm
  • 30m, -130 dBm
  • 20m, -130 dBm
  • 17m, -130 dBm
  • 15m, -129 dBm
  • 12m, -133 dBm
  • 10m, -130 dBm

The basic obvious thing from this test is that at lower frequencies the RX pre-amp is not as sensitive, and the 60m band dip is caused by using the 80m bandpass filter, instead of it’s own, dedicated one.

Local Oscillator Leakage

Next on the list was checking the LO leak to the antenna port, using my Spectrum Analyzer, results as follows:

  • 80m, -83 dBm
  • 60m, -73 dBm
  • 40m, -72 dBm
  • 30m, -66 dBm
  • 20m, -63 dBm
  • 17m, -62 dBm
  • 15m, -58 dBm
  • 12m, -61 dBm
  • 10m, -61 dBm

Again, some quick conclusions – there is more than 20 dB difference between lowest and highest band, so most probably the leak is capacitive breakthrough, instead of design related. Also the worst possible value of -58 dBm converts to 1.6 nW of output power at the antenna socket, so even if we leave it like this, it is not something to loose sleep over, specially on our noisy HF bands where noise level is usually between S5-S9.

Still, i wanted to try few things. One suggestion was to completely rework the RX mixer into double balanced mode, but as no schematics with detailed practical measurements was available, only some pdf with lots of theory and simulations, i have decided to skip this option as it would be too much work, with not guaranteed result.

Another idea was to use low noise MMIC monolithic amp, there are some nice chips similar to the AD one i have selected for the v 0.5 TX mod, but with much lower noise value and again with built in bias circuit. The main criteria would be noise figure and reverse insulation.

But even before starting of the selection process, i needed some extra measurements. For example, the RX mixer is sensitive enough, but there was a need for buffering to reduce LO leak and to compensate the losses in the BPFs and LPFs, some amplification was needed.

Filter Losses

Without going into detail, after insulating some stages, i was able to measure the loss in the LPF/BPF/antenna switch chain per band. It averages at 3.88 dB.

RF Pre-amp gain

Measuring the gain of the BFR93A gain in the v 0.5 configuration is not exactly an easy task. I was too lazy to build it on small proto PCB, so what i did is remove R41 and R42, then feed pin 1 and 3 of the transformer T1 to the Analyzer, then RF signal generator was fed to the input via bypass of R4 in the BPF. Again i did measure gain per band, but as we need average value, it comes to 9.21 dB. I did not insulate the center tap of T1, so we can safely assume that the uniform gain value is about 10 dB.

How all fits together

So we have an idea what to look for: MMIC chip of min frequency at DC, gain at about 10 dB and as much as possible reverse insulation figure. Also interesting conclusion – out of the total RF pre-amp gain we have only 6 dB for amplification, while the rest is to compensate for filter losses.

My selection

After using the above criteria I have selected three chips that were possible match for what was needed and were available for immediate order:

  • TRF37D73IDSGT, Gain 10 dB, Noise 3.25 dB, Reverse Isolation N/A, package WSON-8
  • HMC476SC70E, Gain 10 dB, Noise 3.9 dB, Reverse Isolation 18 dB, package SC-70
  • BGA416E6327HTSA1, Gain 14 dB, Noise 1.6 dB, Reverse Isolation 60 dB, package SOT-143

So all those chips looked nice on paper and used similar connection method, more or less the way the current BFR93A is wired (on purpose).

an070_fig-3

Lots of soldering

There was no other way than removing the RF pre-amp components, installing a small socket and building a replacement board for each amp. It was a lot of work, but it had to be done to have a comprehensive result from the test.

chips

It is worth mentioning that you need to be be brave to do this prototyping work with WSON-8 package. It is not easy to imagine how small it is, it basically fits onto standard 2.54mm PCB pad and have 9 terminals to solder!

The results

My plan was to post detailed results per band for sensitivity and LO leakage for each chip. But unfortunately non of the chips scored any better than the current amp with BPF93A. It was a nice reminder why i have chosen it few years ago. Overall the HMS476 had reverse isolation of 15 dB worse than the BFR93A and the rest of the chips, I was not able to see 60 dB isolation for the BGA416 chip, and it’s gain was very low, probably around 0 dB. The only chip that was impressive was the TRF37D73 – it had nice uniform gain across all bands, average sensitivity resulted into -130 dBm and reverse isolation was not worse than the BFR93A configuration. Obviosly the results were bit skewed due to using extra PCBs, sockets, i would believe this chip would perform even better if soldered on the main board with careful layout.

But..

Off course there is always a but, and this time was the package. This thing make the SC-70 package we used before for the Schmitt Triggers look massive! I am sure the pick and place machine in the factory will handle it without problem, but there are many OMs who choose to solder their RF board by hand.

Time for decision

It might sound cool to use modern chip, but i didn’t see a reason to switch from the current design. Instead i have decided to come back to my previous conclusion about capacitive breakthrough causing the LO leak. So i decided to take another look at the schematics and PCB layout and search for a simple solution. Few things caught my eye:

  • R36 connected to the collector of Q1 might be good as negative DC feedback, but can contribute for poor reverse insulation
  • All MMIC chips datasheets were insisting on 1 uF tantalum in the Vcc bias circuit, we don’t have it here
  • C75 blocking capacitor needs to be close to the center tap of T1, unfortunately i have placed it by mistake far away, next to the mixer bias divider

The mod

So the mod is as simple as cutting the R36 trace that leads to the Q1 collector, wire R36 to the R35/C65 junction, install small 10 uF cap on top of C65 and move C75 next to the T1 center point.

mod2

And here the modified schematics:

sch

The final result

Now the important question, is this mod any good ? Here the results of my test:

  • 80m, MDS -133 dBm, Lo Leak -87 dBm
  • 60m, MDS -125 dBm, Lo Leak -88 dBm
  • 40m, MDS -132 dBm, Lo Leak -86 dBm
  • 30m, MDS -140 dBm, Lo Leak -76 dBm
  • 20m, MDS -134 dBm, Lo Leak -68 dBm
  • 17m, MDS -135 dBm, Lo Leak -71 dBm
  • 15m, MDS -130 dBm, Lo Leak -66 dBm
  • 12m, MDS -130 dBm, Lo Leak -67 dBm
  • 10m, MDS -127 dBm, Lo Leak -66 dBm

There is an obvious 5-10 dB improvement in the LO leakage values, with existing v 0.5 PCB and simple HW modification. The increased sensitivity must be attributed to removal of the negative feedback, so some changes in the base biasing might be needed to reduce gain (R36/R37 values).

Again as all other mods, feedback, comments and ideas are welcome. And this mod is obviously completely optional.