Thursday, 15 March 2018

1296MHz or 23cm Beacon

A nearby Ham, and a good friend of mine, VK6KDX Jack is working on an EME station for 23cm. Locally our 23cm beacon has been off air for eons. For testing purposes Jack needed a distant signal source. Here then is the 23cm beacon, measuring 10cm x 5cm, under construction:



The beacon is now complete and tested. The ident is in CW every 60 seconds. The PLL chip is loaded at start-up by a STM8S1003F, or STM8S003F, micro running the fantastic STM8 forth with more details here. This micro also gates the amplifier chain to produce the morse ident.

I used parts I had in the junk box. The PLL was recovered from some TV modulator boards I have. The varicap came from another TV demodulator board that I had. The mmics were all recovered too. Except for the final mmic. I had planned to use the AH1 mmic shown in hte phot above which is a 100mW output device. However, both my recovered mmics were broken. So I used an AH101G instead which is a 500mW device. The standing current of 200mA from an 8V supply means 1.6watts is being dissipated which my board wasn't designed for (the AH1 is only 0.75W ). To solve this I used some copper tape to better connect thermally the two sides of the board and create a small fin. I also attached a heatsink to the opposite side of the board. So far, so good. The device hits 75 degrees Celsius and should survive the abuse it will receive.

I'll just have to buy some AH1's and either retrofit one, or build another beacon up. I have a lot of boards left over. Please contact me if you want one. I can furnish a partially populated and programmed board for you.

The code can be found here.

Regards
Richard VK6TT

Sunday, 4 March 2018

40m Direct Conversion Receiver - Completed

It doesn't matter how good something appears on the test bench, it's the on-air baptism that counts. This weekend there was an ARRL contest. A proper baptism of fire. So imagine the smile when I turned the receiver on with a dipole connected. Wall to wall signals. North American stations, JA's, VK's.  They were all there. Strong, loud and clear.

This morning, Sunday, I listened to the local new broadcast on 40m. I could hear every station that checked in. And I could just cope with the strong signal of Chris VK6JI who runs the broadcast and call-backs. Chris's station is perhaps 5km away so it is a full scale signal on every radio I have. I hope to simultaneously  record the call backs one day, my homebrew receiver on one channel, my IC746pro on the other. I know there will be a difference, but not much.

Now that I have the bugs of this approach sorted out I'm using one of the commercial boards for an 80m version.

There is one thing that I learned from this version: how a voltage inverter can cause trouble. I used an ICL7662 to create a negative rail for the op amps.  What I failed to catch until I tested the unit was the ripple from the ICL7662 was large enough in amplitude to upset the squelch circuit. That was my fault for not bypassing the V+/2 rail the squelch used. One capacitor and problem solved.

Now all of my design goals have been met.
  • lots of volume from a speaker since headphones don't work if you're moving about the workshop while you listen to a net,
    • Passed - I can easily listen to a net while moving around my very large workshop.
  • no audio instability, 
    • Passed. Micro-phonics solved by using tantalum's instead of MLCC's in certain parts of the circuit.
  • no hum, 
    • Passed.
  • good AGC so you are not jockeying the volume control, 
    • Passed
  • good audio filtering
    • Passed
  • a squelch. 
    • Passed.
That's 6 from 6. I'll finish the 80m version soon but it's time to focus on updating the synthesiser and transmitter to accompany this receiver. Then I'll have a working homebrew 40m transceiver again.

Complete schematics except for the squelch. I'm happy to pass on details of the squelch circuit privately. But as far as I know it is copied from a Codan radio so I'm mindful of copyright.

Regards
Richard VK6TT

Sunday, 25 February 2018

40m Direct Conversion Receiver - Testing successful

Today I finished assembly and correction of a few minor mistakes of the receiver. The end product performs really well on the bench. In summary:
  • about 80dB of dynamic range
  • no instability whatsoever, even at "crazy" loud volumes
  • no microphonics
I'm looking forward to on-air testing this week.

Regards
Richard VK6TT

Monday, 19 February 2018

40m Direct Conversion Receiver - Yes, I'm doing another one!

OK, so I saw a blog post by someone who suggested they were getting a batch of boards done at a great price. I won't name them because their bargain was way too expensive once freight was included. But by the time I discovered that it was too late, I'd made the mental commitment to one final, best I could build version.

Since then I've sat on my hands for almost a year. But this weekend I needed a break from the maths of loop filters and modifying 10GHz equipment. I wanted something I could do without much thought.  And since all the planning for this was done around a year ago it seemed like it was time to start.

I had wanted to see if toroids would offer a significant advantage over the tuned inductors in the RF bandpass filter.  While the existing filter works really well,  I considered moving from adjustable inductors, with a Q of perhaps 70, to toroids with a Q of say 200.

The tighter filter passband became less attractive when I worked out the maximum inductance I could wind on the toroids was around 1uH. A larger inductor has wire too fine to allow for compression and expansion of the coil turns to tune the filter. The advantages of the higher Q are offset by the lower impedances within the filter and the net result is not much difference. Had I had a larger toroid then things would have been different. So I will stick with the existing filter and this iteration will really be about incorporating all the modifications I made on the fly.

Here is a piece of advice I agonised over putting to print. I'm not comfortable denigrating other people's designs. But I was gob-smacked to see someone recommend an AM broadcast filter for a 40m receiver as a way of fixing receiver overload from short-wave transmitters. I'd like to know how that works. Because I can see no way that it can work. Remember, AM broadcast signals are around 1MHz. Those pesky short-wave signals run from 40m upwards to perhaps 16MHz.

And another matter that galls me is people still promoting simplistic designs as being worthwhile. If the project doesn't have at least a double tuned band pass filter between the aerial and first mixer then it's a glorified crystal set. I do wonder if those that propose the "easy" way know what they are doing.  For all the effort that goes into building a receiver reward yourself many times over with another 1% of effort and put in two or three extra components in one of the most critical parts of your receiver.

Here's a picture of progress to date. I hope to have this finished by the end of the month. Then I'll think about what to do with all the spare boards. Any takers??

Regards
Richard VK6TT


Monday, 12 February 2018

EM 10.5-10.68GHz microwave modules - Mods to use on 10.368GHz - update

The first RX module which down converts 10GHz to 144MHZ was finally finished today. Initial results were disappointing. However, after some modifications to improve the noise characteristics of the first local oscillator everything is running nicely now.

The conversion takes a while but the result is a sensitive down converter suitable for SSB and FM use. I'm not posting full details of all the changes required at this point. Please get in touch if you'd like to know more and I'll consider blogging further details.

In the meantime I have a few more modules to convert. The RX module has the following model identifiers:

em                   FM RECEIVER
10.5-10.68 GHz, Synth
P/N 036D                    S/N 111xxx


Wednesday, 17 January 2018

EM 10.5-10.68GHz microwave modules - Mods to use on 10.368GHz

This started out as what appeared to be an ambitious project but it is so far straightforward. I was fortunate enough to be given the chance to modify a few sets of the FM Tx and Rx boxes for ham use. It transpires that just about every set of modules is the same but different! They are all broadly similar but subtle variations exist so if you have some of these modules and yours don't match exactly then this shouldn't deter you.

Here's what I have done so far:
  1. Used a cheap logic analyser to check what was being loaded into the PLL chip by the micro. 
  2. Inserted my own micro to control the 2.5GHz PLL
  3. Achieved lock on several TX and RX units
PLL chip
The units I modified had the LM2326 PLL chip. It has a different data format than the LM2325 chip I saw on the units I have not yet modified.  In a few words the 2.5GHz PLL uses a 250KHz reference frequency. It is multiplied by 4 on both tx and rx units. On the tx units it is mixed with the IF, presently 69MHz but I am modifying this to use a 144MHz IF on tx. On Rx the multiplied signal is mixed with a 692MHz oscillator down to a 140MHz IF. At present I'm leaving this oscillator alone since the saw filter appears wide enough to be serviceable with a 2m IF.

Micro
Once I knew what the existing data stream was it was relatively simple to work out the data stream needed to load the PLL chip with data to put it on the desired frequency. See the chart below for an overview. I used an ATTINY13 and a few lines of assembly to do this.

Achieving lock.
This should be straightforward but sometimes the soldering proves difficult due to the heatsinking the board has. I moved the short until the PLL locked with around 4V-6V on the tuning voltage link.

I'll post some more pics soon but for now the following may prove useful.

Regards
Richard
An unmodified receiver showing what is required to modify for 10.368GHz use
Details of how the IF signal on Tx gets to the mixer diode for up-conversion to 10GHz
Data loaded into PLL chips to shift to 10.368GHz

Thursday, 5 October 2017

Lead Acid Batteries - Measurement of Internal Resistance - Results

It's been about 4 weeks now and some observations of the impact pulse conditioning has had on internal resistance and perhaps battery capacity are warranted. More importantly, the need for a more rigorous approach has become apparent.

Over that last 4 weeks the measured internal resistance has fallen from 264 milli-ohms to 175 milli-ohms. While the reduction in the internal resistance tapered off after about 10 days, there is some support for the battery's capacity having increased as well. At the start I was recharging the battery after 3 days on the pulse conditioner. Now it runs for 5 to 6 days before I have to recharge the battery.

So my initial observations are that the pulse conditioner is beneficial. However, there are several issues with the approach and the results cannot be construed as anything other than weak support for pulse conditioners.

The biggest drawback to the approach is the lack of a control battery. With a second battery I could cycle one on the pulse conditioner whilst the other was cycled on a static load. If the static load battery showed little, or no improvement, in internal resistance then there would be much stronger support for the pulse conditioning approach.

Another drawback stems from the lack of automation. I still have to manually changeover the battery at each step of the pulse, charge, measure cycle. So the time between measurements is not constant and the level of charge and discharge varies from one cycle to the next. I clearly need to automate the cycle and let it run unattended.

The final obvious shortcoming is temperature. We are moving towards summer and the average ambient temperatures have increased over the last month. The temperature change could be influencing the results either directly via battery chemistry somehow, or indirectly by it's impact on the voltage regulator which serves at the voltage reference for the D2A conversion. A temperature controlled testing environment would be useful to remove another source of potential error but that is beyond my reach at present.

The biggest obstacle to the full automation with the W1209 board is the need for an additional two digital outputs. I have considered two approaches. The first is using the + and - keys as both inputs and outputs. That would require some careful soldering to insert a resistor, say 2k, in series with each switch. The second approach is a 4017 counter clocked by the pin driving the relay. Then each of the decoded outputs for 1-3 from the 4017 driving a relay to perform each step of the pulse charge measure cycle.

I'm leaning towards the first approach since I don't know if I have a 4017 in the drawer and the first approach avoids any ambiguity over which step the cycle is in.