He want to be able to use his RTL-SDR receiver with this radio.
The repair was nothing interesting. The Rx/Tx relay was fried and the RS signal was not present on the first batch of switching diodes.
The relay was changed with a similar one and the fried PCB was repaired with a wire directly from the Toshiba high current driver.
On the other hand, the asked mod was a real challenge!
The first IF on this radio is on 47.055 MHz so it's suitable to use it with a low cost RTL SDR but the signal is very low before the crystal filter. It's better to have the signal tapped before the filter. Why? Well, the answer it's obvious: to have a whole 2 MHz to enjoy!
So, first, I tried to use the G4HUP schematics (PAT) but it didn;t work as expected. The signal was very low, maybe because I tapped the IF signal from a different point or maybe because my J310 are fake :-).
This is where I put the IF tap:
It's a low impedance, low signal point so I needed a good RF preamplifier.
Listening with a RTL-SDR produced usable signals only on strong signals so I was looking in my little boxes after some suitable components. I remembered that I have somewhere some Analog Devices AD8008 for a "never started project" and I decided that this is my path!
So, I quickly draw a schematic, I also drawed on a PCB and put it to work.
Here is the schematic, based on the application note from Analog:
Everything was in SMD because I was looking for a small footprint, but the result was nice (at least for what I needed:
I also found some metal sheet to made a custom shield case.
I didn't save the results from the IFR sweep analyser; the amplification is around 12 db at 5V and is very flat, which is what I was looking for! At 9 V, the amplification is around 17 db! Good to know!
The 5V for powering the amplifier is taken directly from the Q1034 on the SMD side of the "RF" PCB board. I didn't took it from L 1012 as is shown on the G4HUP page because there was some problems with that path and I didn't want to stress the PCB. So I took the 5V directly from the TD6278P SMD circuit (Q1034), on pin 12 (RS).
This 5V is present when the radio is on receive only and when transmitting is 0V so the preamp is protected from accidental high RF at input.
As for the output, I had a nice BNC connector with a small RG316 cable, just asking to be on the rear side of the Yaesu FT-840!
This is the final work:
And here is a small youtube movie with the SDR at work:
13 comentarii:
Very nice work! Is there any way to make the SDR shown in the computer screen the same frequency as the Yaesu FT-840 display? Thanks.
And also, I would like to ask you where are the other IF points (about 8.215 MHz I think) in the main board of the FT840, since perhaps signal amplification is not needed at those points. Thank you in advance, and keep up this great work, 73 from Argentina!
You can tap wherever you want as long as you provide a high impedance tap. A low capacitance will do the job but after that you need some amplification because the signal is drastically reduced. High impedance is needed because a low impedance tap will alter the normal function of the tuned circuits in the IF.
I think there is a settings somewhere in the #SDR for that!
Does this card fit the MDSR/SDR application:
https://pt.aliexpress.com/item/AD8008-650MHZ-dual-channel-bandwidth-current-feedback-amplifier/32750098334.html?spm=a2g03.search0305.4.6.onXRyO
Comes assembled
pu4asl@yahoo.com.br
At first glance will do but I noticed that the board need +/- DC so some sort of DC/DC for the negative voltage will be required... That might ask for complications on conducted RFI and also for the Rx/Tx switching to disable the board while transmittimg... :-) 73 from Lindau, near Friderichshaffen 😆
Hi, I am building the circuit, and I see a difference between the schematic and the actual circuit you made.
In the schematic, after pin 7 of the amplifier, the signal goes through a capacitor, then a resistor, and then to the output.
But in the circuit you made, the signal of pin 7 goes first through a resistor, and then to a capacitor, and to the output.
Which one did you use? The resistor and then capacitor, or vice-versa?
Thank you very much!
Hi YO3HJV,
I have built this amplifier with standard components (not SMD), following your pictures, and it worked great right away! Great work! I managed to get a small circuit size, but it is not as compact as yours.
I used a piece of the coax provided in the SDR dongle between the tap point and the amplifier, and another piece with the mcx connector to exit the radio in the back and go straight to the USB dongle.
Then, I powered up my FT840, opened the SDR application, I things went smoothly well! I can see and hear all signals, weak and strong ones. The total bandwidth is almost 2MHz, and in TX mode the amplifier shuts off protecting the circuit.
I'm using "gqrx" and "CubicSDR" in Linux (tested on Debian 9 and Ubuntu 18.04) and everything works fantastic.
Thank you for posting these instructions and for all your comments, keep up the good work
73! Alejandro Albanesi LU3FAU
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You cand put whatever you want first, the signal will be the same. So, capacitor-resistor or resistor-capacitor are equal in terms of the signall passing by.
Man, I am really glad you didi it! I see your first message is more than 2 years ago and looks to me that this was something of great interest to you! It's good because I do not own the radio; I did the mods for some fellow ham and returned the radio. I miss the pictures from the other side of the PCB board where the tap is. Maybe yo did some and post a link to them to be a visual help for others. Again, congrats and thanks for sharing the good news! 73 de YO3HJV
Hi Adrian, sorry for the late reply.
Yes, I have some pictures of the other side of the PCB that can help others to clarify things. I can send them to you, or I can try to post them here as an update of the project.
The capacity to see the waterfall of signals below and above the frequency displayed on the radio is very useful, and also a lot of fun. You can hear 2 different frequencies at the same time, one on the radio, and the other in the SDR. An extra feature is that the signal on the SDR has not been filtered by any other IF filter, so one can play with filter shape and bandwidth. With some practice, one may be able to get on the SDR a very sharp receiver to hear even very low stations.
I'm even studying the schematics of my VHF radio, the little Yaesu FT-1802. I found the IF crystal filters (21.7 MHz I think they are), I might as well give it a try one of these days.
73,
Alejandro LU3FAU
two questions. does anyone make a kit for this? and what mechanism turns the receiver off during transmit?
thank you much
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