System Noise-Figure Analysis for Modern Radio Receivers (link)

One guy at MAXIM Integrated has published an interesting paper about System Noise-Figure Analysis for Modern Radio Receivers. There is a little math and examples of measurement on DSB and SSB receivers.


And don't forget to check their Tutorials area (linked in the page above), with other interesting writings.

All well worth a read.

Transverter sequencer with Arduino

Even if not explicitly mentioned, I decided my 70 MHz transverter needs a switching sequencer. Any existing solution I could find online uses either discrete components and ICs or a microcontroller I cannot handle (PICxx series).

Given the widespread availability of Arduino derivatives, their low cost (< 10 USD) and their extreme flexibility, I preferred to go down this route. Besides, an Arduino board allows for easy firmware changes (just plug it to an USB port of a computer!) and can be programmed for CAT communication with the FT-817 and alike (FT-857, FT-897, FT-100).

The sequencer code uses a level change interrupt (on INT0) to monitor the PTT line output on the CAT port ("TX GND"). This line goes to logical low level when the PTT is pressed

Once a transition RX-to-TX is debounced and detected, the main loop switches in sequence the IF input attenuator (not needed in RX), the transverter PTT and enables RF output on the radio (pulling down the "TX INH" line). Small delays allow for each change to be applied by the involved circuitry, being the relay the slowest to react. The TX-to-RX transition reverses the sequence described above.

At startup the sequencer inhibits RF output and configures itself in RX mode.

This is not rocket science at all.

If another output is needed it is just a matter of adding few lines of code. An extension to the code should allow to switch the RTX to the IF center frequency (29.200 MHz USB in my case) as well as make sure the FT-817 RF power is the lowest possible. Then drive a status LED, monitor the transverter PA temperature, ...

Transverter 29/70 TX'es fine

In a careful hurry I checked the 4 m transverter if it is suitable for SSB traffic.

Since it must be driven with 100 mW, a 7 dB attenuator was built so that FT817 500 mW would not fry the mixer stage. A "real-world" 7 dB Pi-attenuator consists of a 2W 120 ohm resistor across, 47 ohm straight, 120 ohm across. I used standard carbon resistors and a quick check with MFJ-259 reported a 1.2:1 SWR max up to 170 MHz.

With full manual switching I listened to my whisper transverted to 70 MHz on a different receiver and it sounded good.

This was the "GO" signal to start planning for a sequencer (probably embedded into one of my I-F-R-K accessories) and a suitable box: this time I will have to do the mechanical work!!

Near Field Communication recycling

I have attended a speech at the local Linux Day 2013 about Mifare Ultralight NFC ("Near Field Communication") cards and implementation bugs that render the system vulnerable to some "attacks" (now fixed, too late ;-) ). They were at DEFCON 2013 too.

These cards operate at 13.56 MHz and can carry some data as well. They are being widely adopted as electronic tranportation tickets, access badges, ... Since they come with a factory programmed unique ID, some DIY recycle is possible at almost no cost.

The speaker has described the data memory map of those cards and explained how some bytes are one-time programmable: once a bit is set to "1" it cannot be returned to "0". That is how some transportation systems keep count of used/available tickets on the card. The data area remains rewritable and the unique ID cannot be changed.

Reading (writing) these NFC tags is pretty cheap (about 20 USD@2013) and can be done with a computer or an Arduino(-like) board. Of course some sort of software is needed to interact with the reader hardware and the card.

Since I am curious about this technology I have ordered a card reader/writer at least to see what kind of home applications can be tought of. A poor man's presence detection, for example, or an authorization system for you-never-know-what.

It is QRP afterall :-)

Transverter 29/70, RX current consumption

I found no mention online about current draw of the Ukrainian 29/70 transverter: my specimen drains about 24 mA at 12V in reception.

More to follow, especially a TX test once I found what is a suitable heatsink: just the containing box (yet undefined) or something larger?!

If everything works, the construction of a simple sequencer will follow. Most probably based on one of my (I)FR(K) projects.

Transverter 29/70 arrived and switched on

The "ten to four metres" transverter has arrived from Ukraine. The very first impression is that it is indeed SMALL! A picture-to-real comparison with the original 10-to-2 m transverter shows that coils are different, so it has somehow been retuned to 70 MHz. Look at it on a 5€ note:

First on-air test was the reception of the local beacon. I started closing the output on a 50 ohm load "just in case" (yes, I know the load shown is not able to handle more than 0.5W). Then about 4 m of RG58 going to the FT817 tuned on 29 MHz: the local beacon was coming through S7, with the dummy load as antenna! Actually, dummy load or not, it made no difference.

The transverter circuit is very very very simple (and I have spotted one error on the included diagram), but the RX chain is hot. Using the same coax length on the IC706 tuned to the beacon frequency resulted in the same signal strength. So, apart from RF leakage, it is a (good?) sign of life.

Packed up RTLSDR

A local HAM agreed to compare his 23 cm transverter with the RTLSDR in order to see if the little stick can be competitive in certain situations, like a contest. So I needed to do some adjustments to my typical home setup.

First of all, his cables terminate with N connectors. I initially though of changing the short RG174 cable end into a female N, but quickly realised how weak the whole system would be considering the weight of N and in-flexibilty of thick coax. I needed to protect the dongle with a box.

I picked a metallic chocolate box I kept aside some months ago and fitted a flanged N female. While I was at it I also shortened the RG174 cable to what was strictly necessary (1 metre to 10 cm): these two actions resulted in at least a couple of dB gain (or ... "less loss"). Note that 1m of RG174 at 1300 MHz looses 1 dB.

The USB hole is not something I am proud of, but it was realised in less than 5 minutes, so it is acceptable for this experiment. If the on-the-field comparison shows RTLSDR is a decent performer I will consider soldering directly into the PCB both the USB cable and the N or BNC female head.

Transverter 29/70

While searching online for parts for a 70>23 cm tripler I came across an auction for a 10m to 4m transverter board. Just the fully assembled board and an A4 sheet with instructions.

It is supposed to output 5W on 70 Megs with 150mW drive on 29 MHz. It should be all-mode. It needs a heatsink, housing, wiring, IF attenuator and RX/TX sequencer.

Looks like a good companion for the FT817. Time will tell, since I bough one of those.

Edit: (At the time writing there is one board left in Ukraine)