The mcHF HF SDR Transceiver - 4

As I am still waiting for the MOSFETs to arrive, thought of sharing some technical details on the mcHF.

Features:
  - A standalone[PC not required] compact embedded HF transceiver
  - 2 MHz to 30 MHz Continuous frequency coverage
  - Operates on USB, LSB, AM and CW
  - 2.8 inch color TFT LCD
  - Four rotary encoders and 17 buttons for easier operation
  - Fast and fully electronic RX/TX switching
  - Two USB ports – for PC control and external keyboard
  - Two temperature compensated oscillators/clocks
  - Four digital filters – 1.8kHz, 2.6kHz, 3.6kHz and 10kHz
  - Built in Iambic Keyer that supports Mode A and Mode B
  - Large 48 kHz spectrum display
  - Variable tuning steps

Hardware Specification:
1. Processor - STM32F407 from ST Microelectronics.

     - Core: ARM® 32-bit Cortex®-M4 CPU with FPU

     - 168 MHz, 32 bit
     - 100 Pin package

     - 3.3 V operation

     - 1 Mb flash memory

     - 3x12 bit A/D converters

     - 2x12 bit D/A converters

2. Codec - WM8731 from Wolfson

     - 28 Pin package
     - 3.3 V operation
     - ADC, DAC sampling frequency - 8 kHz to 96 kHz
     - ADC SNR 90 dB
     - DAC SNR 100 dB

3. RF Power Mosfet
     - 2x RD16HHF1 RF MOSFET from Mitsubishi

4. Local Oscillator
     - Si570 from Silicon Labs
     - 10 MHZ TO 1.4 GHZ I2C PROGRAMMABLE XO/VCXO
     - 8 Pin SMT
     - 3.3 V operation

5. SN74CBT3253C Tayloe detector as a direct conversion mixer

The mcHF HF SDR Transceiver - 3

In my last post, I had mentioned about the issue of fuse getting blown off. The cause of the problem was traced to PA stage and I did a workaround to skip the problem and continue with the receiving part. As a workaround,I had removed the RF Choke through which power was being supplied to the final PA stage, so that the PA stage was isolated.

Now that the receiving is working well, I started thinking on the first issue i.e blowing fuse issue. This time there were lot of suspects in my list :). The output transformers, the SWR transformers, driver transistors and many others. 

The problem is, when ever I provide dc supply voltage to the PA stage, the transceiver draws a hell lot of current and the supply voltage reduces to as low as .68V from 12V. And the transceiver won't power on. And if the power connector is removed, it blows off the fuse as well. 

There can be many possibilities for this issue like wrong connection in output transformer, shorts in any of the transformers wound on Binocular cores, High bias voltage reaching the MOSFET gate which in turn could conduct all the current from Drain to Source, any damaged smd components in the PA stage and many others.

The first thing to be done was checking for shorts in output transformers after removing from the PCB. But couldn't find anything wrong in the transformers. Next I checked the bias voltages to the MOSFET gates which came from the LM2931 IC. That too looked OK. 

What now?? There could be some RF voltage coming into MOSFET gate which is driving this MOSFET crazy. The RF voltages will go undetected in DMM(Digital Multi Meter). So I connected an RF Probe and checked for the existence of any RF voltages in the MOSFET Gate. RF voltage in Gate=0V. So these experiments continued. I never suspected the PA MOSFETs , because they were bought from ebay.com from US and brought to India by one of Vimal's(VU3CPE) friend. (Vimal a friend of mine and myself are sourcing the components together and building the mcHFs individually)

Also Vimal had tried replacing his MOSFETs with one which was bought sometime later(We had bought the MOSFETs twice, because of the first package getting lost during transit and later on getting back the same. It's a long story:). Anyways we ended up having two sets of same MOSFETs, Toroids etc.)

So all these things made us believe that MOSFET is not the culprit. 
Experimented many things, but all resulted in high current drawing and shorting the supply voltage. With all the possibilities exhausted, I turned to MOSFET. The MOSFET used are two numbers of RD16HHF1 made by Mitsubishi. I took out the MOSFET from the circuit and tried to check the Vgs vs Id(Gate voltage vs Drain current). 

Lets go through some basics of operation of a MOSFET.
MOSFETs are nothing but valves,in which the current flow can be controlled by varying the Gate voltage. So till the Gate voltage reaches a certain value, the MOSFET will remain non-conducting. This value is know as the Threshold voltage or Vgs(th) or simply Vth. Usually this would be somewhere around 3-4 Volts. That means till you supply at least 4 Volts to the Gate, the MOSFET will remain off. This region of operation is called "Cut-Off" region. Once you increase gate voltage above 4 Volts, only then the MOSFET will start conducting. 

Back to our MOSFET experiment. I needed to check how our MOSFET was behaving in the cut-off region, i.e. when Vgs<4v. I had to set the Vgs to '0'. So shorted the MOSFET Gate to the Source and connected it to a negative of my power supply with voltage set to 10V. The power supply is a small one capable of sourcing only about 1.2A. The DMM was kept connected to the power supply. As soon as I connected the positive lead of the power supply to the Drain, the supply voltage dropped to 0.68V almost shorting the power supply. Repeated the experiment, this time with the DMM connected as an ammeter in series to check the current draw. The whole of the current which the power supply could source was flowing through the MOSFET. I couldn't believe this :) there seems to be no effect of the Gate Voltage(Vgs) on the Drain current(Id). And this confirmed that the MOSFET was the one shorting the entire circuit.

 Meanwhile, Vimal(VU3CPE) got his MOSFETs replaced by sending his completed SDR Tranceiver to VU3GEK and it is transmitting fine now. The video of the same is available in YouTube. Also Vimal is getting a few number of RD16HHF1 for both of us from VU3GEK, so I m waiting for the new MOSFETs to arrive. I am experimenting with IRF510s till I get the RD16HHF1s. The Drain, Source pins are reversed for IRF510. I have used IRF510s in past for the BITX transceivers that I had built. The IRF510 being a switching MOSFET, will nowhere match the performance of RF MOSFET RD16HHF1, but still, something is better than nothing...at least for the time being. :)

The mcHF HF SDR Transceiver - 2

There had been a long gap after the completion of the mcHF UI board to start with the RF board soldering. Mainly it was for the time required to source all the components needed for the RF board.So to cut it short, I started with the RF board soldering and completed soldering about 90% of the components using hot air soldering. I left the smd inductors, relays to be soldered in the end using conventional soldering iron since hot air would melt their plastic bodies. And the hand wound inductors were the last parts to be soldered into the RF board. All done in about 2 weeks. Now, the real challenge :) Testing it and making it work.

Connected the board to a limited current power supply. And pressed the on/off button. But nothing happened. It didn't power on. :((Before soldering the RF board components, it was working..see the photo and video in my last post.)Found that the SMD fuse in the power supply section was blown out. And the worst part is, I didn't have much fuse left for experimenting. There was a Schotky diode for reverse polarity protection. It became my central point of suspect along with few others. After taking the suspects out of the RF board, it was found that they all were totally innocent in this issue.

The next suspect was the transmit side with the PA consisting of 2 nos of RD16HHF1 MOSFETs rated for 16W 3-30MHz, Torroids, LPF, BPF stages, Antenna switching etc. So the best thing to isolate the stage was to cut the power to that stage. The 12V supply comes to this PA stage through an RF choke RFC8 which is a single turn 24SWG copper wire through a small binocular core(BN43-2402). I wanted to make the receive part working before going for the transmit. I removed RFC8 cutting the power to the PA stage. Replaced the blown out fuse with a new one.And there it is, the Rig powered up showing the spectrum display. So the fuse blowing issue was isolated to the PA stage. Thought of checking it later, once the receiving is found to be working.

Connected my speaker...And guess what...there was no audio at all.I increased the AF Gain to max, RF Gain to max. But still no audio.
I traced through the DC voltages for the ICs in the receive stages and everything seemed to be fine. Checked for any soldering bridges in the processor for the digital signal input/out put pins. But that too seemed fine. Checked CODEC, Switching IC, Speaker amp. Again here my suspect became the SMD Audio amp, the LM386M-1.
The DC voltages seemed to be fine. To check if the Audio amp was working, I isolated it from rest of the circuits by removing the coupling capacitors. Connected audio from my Samsung Tab to the input of the LM386M-1 and played a song. There came crystal clear audio from the speaker. And there goes LM386M-1 out of the suspects list.

Next was the Codec IC, WM8731. After removing the isolation made in the LM386M-1, I did the same thing with the codec IC. Isolated the input to the codec and connected the audio from my tab playing an mp3 at low volume. But this time there is no audio from the speaker. With this I became doubtful if the  codec's working.The analysis continued for two evening sessions. On the third evening,I noticed a strange thing which went un-noticed in the initial analysis.
There was a short between pin 15 and 16 which should not be the case according to the schematic. Across these pins are connected, C22 and C23. C22 being an SMD Tantalum, I thought it must have been blown out. I desoldered it and the short was still there between pin 15 and 16. And the pulled out tantalum was also not shorted. Removed C23 SMD capacitor and the short between the pin 15 and 16 is there no more. :)

Even though the pulled out capacitor didn't give any short while continuity check, I replaced it with a new one after cleaning up the board. May be there was some un-melted BGA soldering paste(mixture of lead and flux in the form of paste) under C23 which lead to this short. Anyways, I didn't go for further testing it. I went ahead with connecting the rig to power supply and then switched it on.
:):):)

The rig came up with audio in speaker :). Connected my 20m antenna to it and searched around the band and rest you can see it in the below video :).

I have built many conventional radios in the past and I feel that the happiest moment is when,you hear some one talking from some where in this world, through the radio that you newly built. And this time also it was no different with this cute little QRP SDR.