Using a D2E-S-DC12V to Switch RF at 6M

 

Initial tests are encouraging. Insertion loss less than 0.3dB and both isolation and return loss around 35dB.

Time now to put it on the 6m radio and see how it copes with 50W of FM.

73's

Richard

New Part - VHF Low Noise Amplifier MXDLN02C

So I was browsing for a high speed op amp and I came across this wrongly catalogued part at LCSC.COM. I am now testing it for a commercial project but it could have uses in ham projects.

But there are some caveats that go with a US$0.05 part:

1. The datasheet could be better
2. This is not a strong signal handling part
3. No details exist on how best NF is obtained

The MXDLN02C is meant to be used as a pre-amp for FM broadcast receivers. The datasheet suggests the noise figure is under 1.5dB and gain is above 20dB from 50MHz to 150MHz with good stability. I wrote to Maxscend as part of the commercial project requesting the S parameters in a table format. The response suggests Maxscend will never be one of my preferred suppliers.

So, in Ham spirit because this is a really cheap part, I simply hooked up the input to the nanoVNA and measured S11. Now I don't claim the nanoVNA is the last word in measurement, or that my test board was perfect. But the results I got suggest Maxscend was way off the mark.

From the Datasheet (2.85V) :

From measurement (also 50-150MHz) (3.3V):

I'm not dwelling on the possible reasons why there is such a disparity. What is important that based on my measured S parameters matching networks were easily calculated in RFSiim99. So, a few minutes of tweaking and I had a return loss of -30dB on the input at 6m and about 20dB of gain.

A similar story on the output port.

Summary:

It's ridiculously cheap.

Useful for 6m and 2m in non-demanding front-ends, still has +13dB of gain at 70cm.

This is not a MMIC so matching to 50 ohms, while easy, will be required

Like a MMIC no biasing calculations needed

Stability concerns limit HF applications

I always advocate using what is in your junkbox. But if you had to buy a device for low level amplification at 6m or 2m then this is what I would suggest.

73's

6m MX800 Tranceiver conversion - update

It turned out that there is a minimum divide ratio for this chip and the only way to use it is to have a much lower reference frequency than 25kHz. I decided that if I was going to modify this for 6m I would use my own synthesised local oscillator design and avoid using a fractional-N PLL.

However, there are moves afoot for VK Hams to gain access to 4m. I'll shelve this project for now since this transceiver would be readily suited to a 4m application. Time to go find another project!

73's
Richard VK6TT

6m MX800 Tranceiver conversion

Rummaging around in the workshop recently I found a cannibalised MX800 transceiver by Spectra Engineering. These appear to be a really well built commercial radio used for repeaters. Lots of fantastic hardware. This was a B series radio covering 70 to 88MHz. We don't have a 4m allocation here in VK so it would need to be modified to 6m to be useful.

Since the receiver board was partially detached from the chassis I pulled it out and brought it down to the house for a closer look. The first thing I did was find a service manual. It showed the front end filter could be padded down to 6m. However, the filter topology was unlike anything I have seen before.

It appears to be  a chain of Pi filters with a series coupling capacitor. I couldn't find any details of the transform used so instead I pulled up the Iowa Hills RF Filter Design software and examined if there was scope to modify this filter to something I could derive values for.

My guess is the inductors on this board were around 60nH with an unloaded Q of 70 based on data I had seen for somewhat similar inductors. While the modification to the layout for a series bandpass filter would be little effort, I quickly established that a series inductor configuration had very little attenuation below 6m so I discarded that approach. No real surprise in hindsight given the small inductors.

Turning to a conventional capacitor coupled bandpass filter I quickly ran up a 3 pole filter with a centre frequency of 52MHz, a bandwidth of 12MHz and standard capacitor values. My initial plan was to run the Local Oscillator on the high side of the received frequency. Attenuation on the image of the local 6m repeater which I would mostly be listening to ( 53.8MHz + 2 x 45MHZ IF) of 143.8MHz was around 35dB. Since there are two of these filters, one before and one after the RF pre-amp, that amounts to 70dB of attenuation.

Which didn't sound like a lot to me if I was transmitting on 2m. My reasonning is 50W at 146.5MHz is 47dBm. Assume this is an ideal transmitter with no phase noise spreading the energy into nearby frequencies. That 47dBm gets attenuated by 35dB before hitting the pre-amp. Again, assume the pre-amp ( an SGA-6489 MMIC) can cope and amplifies this by 20dB before another 35dB of attenuation. Thus the mixer sees a net attenuation of -35dB + 20dB -35dB, or -50dB.

I am guessing but 47dBm from the 2m antenna might result in 0dBm into the 6m feedline. So my 2m transmission is only attenuated to -50dBm at the mixer in the 6m receiver. No big deal if I assume an ideal local oscillator with no phase noise since the mixer is a nice double balanced diode mixer and will probably cope.

But what if the local oscillator has a noise floor that is  60dB below the peak? If the mixer has a loss of 6dB does that mean a 2m signal gets mixed down with 66dB of loss? I don't know but I would be worried that after a lot of work I find every time I transmit on 2m I have to turn off the 6m receiver to avoid the squelch opening.

I've not noticed this issue or de-sensing on my existing 6m radio. I know the existing 6m receiver has much higher Q inductors. It took just minutes to run up a hypothetical filter with a higher Q and a200nH coils and compare it with what I was proposing to build. I concluded the filters I could make using the inductors on the MX800 board would probably suffice.

Comforted by a few minutes work I had a close look at the board. It would appear that by removing all the filter component I could use the inductors and new capacitors with the existing traces.

I am still grappling with the Fractional N synthesize chip. I will continue to look into this but I thought it worthwhile to reinforce my message about filter software with this post. If you're not using the Iowa Hills filter software, a free program, then you should. It took me considerably longer to write this post than it did to model the filters discussed.

Regards
Richard VK6TT