Boys with Toys 2

My last posting was about using my RIGOL Spectrum Analyzer with frequency range up to 1.5GHz to show characteristic of a 10GHz filter. That little test made me wonder if I can use the same Spectrum Analyzer to display a signal in the 10GHz range on that analyzer. Obviously, I have to mix the signal down below 1.5GHz using a transposing signal from another synthesizer.

I have used my 24GHz transverter with 11.88GHz local oscillator, because that signal is conveniently fed to a connector on the transverter's front panel. The same mixer from my previous test (IF range only 0-150MHz) was used for the first test today:

Yes! It really works! My ELCOM on the left side is programmed to 10.805GHz (+13dBm) and the gray transverter on the right side delivers 11.88GHz (+10 dBm). Difference of these two signals is exactly 1.075GHz which is what we see on the analyzer. What we also see is the signal level below -40dBm. Hmm ...

I have tried to "zoom in" and changed the RBW ratio on my RIGOL from 1MHz to 300kHz. Signal still looks nice and clean, just not too strong:

Can we do something about the attenuation? Of course we can. I tried 3 things:

• Find better flexible cables
• Replace flexible cables by Semirigids
• Use different mixer

Luckily, I have bought good sized bundle of these flexible black cables with SMA connectors cheap on eBay. Maybe I could find some better pieces in that bundle? Tried and ... the signal strength improved +30dB:

Wow! This is almost perfect! Maybe Semirigids will make it even better:
 

Hmmm, not too much. Only about 2dB. Surprising, really. These black cables are RG-178 which is OK but not as good as semirigid, at least in theory. Whatever ... let's try another mixer:

Nice! Improvement about 8dB! This mixer has IF range 0-2GHz, but it has apparently been exposed to salty water somewhere. I have bought it cheap on eBay. Lets try better one:

No surprise here, less than 2dB below zero. This mixer behaves as expected. Note that I am feeding it LO signal 11.88GHz about +10dBm and the test signal is 10.8GHz about +13dBm. My RIGOL has internal attenuator of 10dB, so the conversion loss in this case is 13 - (10-2) = about 5dB. That is certainly very acceptable!

My next test will be with a Harmonic Mixer (same thing, but the mixer will generate harmonics from a strong base signal to be able to display higher frequencies, such as 24GHz). But that will be the topic for some other day.

73 Herbert
AF4JF

Boys with Toys

Even at my age of 55, I still feel like a boy when it comes to my electronic (not to mention radio electronic) toys. Especially if my friends keep encouraging me by sending me links to interesting articles on the Internet. Like the document (tnx OK1DXD) where I found following picture:

Nice ... especially after finding dual mixer (two mixers with integrated LO power divider) on eBay:

The seller stated max frequency 18GHz and max IF frequency 150MHz. Well, I overstepped the IF maximum almost 10x and the mixer still worked, so I hope that RF inputs will accept 24GHz.

Now, what to test? Easy pick. I have found a nice waveguide filter on 10567.5MHz:

So, I have located couple of cables, not really suited for 10GHz but at least with SMA connectors, and connected everything together:

Where to get LO signal from? That was easy too - my favorite ELCOM synthesizer provided +13dBm on the frequency of 11GHz:

So far so good ... but will that thing work? I have turned my RIGOL analyzer on and activated the Tracking Generator - and YES, it worked:

Let's try to zoom in to check how steep the filter is:

Pretty good, actually. The passband is flat and only ~40MHz wide. Looks like good filter for general use. With this mixer, I should be able to re-tune it to 10368MHz easily.

Of course, I need to calibrate the "Y" axis. No idea how much attenuation came from the mixer itself. Some definitely was introduced from the measuring cables ... still work to do on my side. I will also try to use the same principle (different Local Oscillator and maybe different mixer) for 24GHz.

73 Herbert

Filters, filters and more filters

Exact, stable frequency is very important in ham radio. Especially if you intend to use digital modes, such as WSJT. I have built a GPS disciplined oscillator to generate 10MHz reference signal and used it in my 1296MHz transverter. It worked nicely ... until the moment when I connected a 40W power amplifier. Wow ... I intend to run 1500W for EME some day, so my GPS should withstand much more than 40W!

What is a solution? A filter, of course. The 23cm band uses 1296MHz while GPS uses 1575MHz. Should be easy to separate.

I have found an eBay seller named "iseeabluewhale" who sends packages signed "GPIO Labs" from Ontario, Canada. This seller offers reasonably cheap filters. One of them is a bandpass for 1575MHz (GPS frequency L1):

 

I have measured the frequency response. Looks quite good, 1296MHz suppressed nearly 60dB:

Time to test the GPSDO with this filter again. I will post the results later.

Second filter that I needed was 432MHz bandpass between my new 10GHz and 24GHz transverters and IF radio. One of them will output IF on- and below 432MHz, the other above 432MHz. Luckily, GPIO Labs make both filters, see the pictures:

Nice filters! It would be nice to find something like that for the 1296MHz frequency. There was one in their eBay listing, but the bandwidth is too narrow for the weak signal portion of the 23cm band:

 

What can we do? Obviously - get a filter elsewhere. Maybe make it ourself! For example using this design from W6PQL. I have made one of these filters (very easy to build) and here is the result:

 

Yes, this filter is quite good. Definitely cheap and easy to make. And the suppression of unwanted signals goes below 80dB!

I have recently used that filter to suppress unwanted spurious signals from ADF4351 synthesizer in the posting below. Quite a game changer there!

73 Herbert
AF4JF

PTT output for IC251a

Friend of mine (Rich, N0PQU) owns IC251a. It is a nice reliable 144MHz all mode transceiver. Rich wants to use it as IF radio for his 10GHz transverter. Problem is that this radio doesn't have PTT output.

We have found nice article (by Scott N0EDV) about installing a sequencer inside this transceiver. However, his technical solution wasn't exactly what we needed, so we designed one of our own.

The IC251a has a RX-TX switch on its front panel. Our first idea was to use that. This switch has +9V on it in RX mode and 0V in TX .. apparently the 9V comes from a switching logic inside the radio and the RX-TX switch grounds that when transmitting. 

We needed something with high impedance and we also needed the logic reversed (switch relay in TX mode), so we designed following circuit:

First half of the OP-AMP, the "O1a", is basically a "unity gain amplifier" with very high input impedance and  the second half, the "O1b", is a voltage comparator. Should the input voltage on the PTT switch drop below ~8V (RX-TX is switched), then the comparator energizes its output, "LED1" comes ON and the optical insulator "OP1" switches its (galvanically isolated) output ON. Exactly what we needed.

HOWEVER

Connection between "PTT" input and the RX-TX switch must be made with a 100k resistor in series, otherwise the "unity gain" amplifier doesn't copy the "0" voltage when switched to TX. This is because of the way how this OP-AMP is connected to the power supply - no negative voltage applied. In the real world, the "-" (inverting) input always has a little lower voltage than the "+" (non-inverting) input, so if you ground the "+" input by PTT directly, the "-" input has nowhere to go and "O1a" goes to output >10V.

We have taken several pictures of the connection points inside the radio to share the idea:

Everything works well on a test bench. We will connect it to the transverter soon and update this Posting with results of that.

UPDATE 

This radio was successfully used for 10GHz contacts on 09/29/2012, details here.

Power Supply for DMC modules 24GHz

DMC modules used for 24GHz transverter (see my other post here) need (+8.4V) and (-5.0V) voltages, where the (-5V) must be applied first.

I have built a simple power supply that honors this requirement. Original idea came from another HAM's web site. I liked this simple solution with a Zenner Diode, so I adapted it for my needs:

First test results were good. Note that output voltage (+8.4V) can be adjusted by changing resistance of R3. In my case, I have used two resistors in parallel. PCB provides enough space for both of them.