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Mehdi

@MehdiHacks

Mar 9

🧵 RF basics: mixers.
A thread for beginners on:
1. What an RF mixer does
2. Understand its datasheet
3. Test its specs

I've picked Mini Circuits ZMDB-24H-K+ for this thread to work on.

1/7

As the name suggests, mixers are used to “mix” signals. But what does “mix” mean in this context?
Mixers have 2 input ports , and an output port.
When you “mix” 2 signals, you end up with multiple signals at the output!
f_out = |f_RF ± f_LO|
So, you have the sum and also difference of the signals!
The output is usually called IF (intermediate frequency)
The main input is called RF (radio frequency)
And the other input that basically controls what happens to RF is called LO (local oscillator)
You can use LO to down-convert RF.
A very practical example is down-converting a multi-GHZ signal down to less than 6GHZ so you can “see” it with your typical 6GHZ spectrum analyzer or SDR.
So, when you hear the terms “down-converter” or “up-converter”, you know they’re simply mixers inside.
[note: in this thread I am treating the mixer as down-converter. Mixers can also work in the other direction (IF to RF) as shown in this screenshot, and work as upconverter. Mixers are bi-directional devices]

2/7

Now, let’s look at the main specs of a mixer:
A very important spec is the RF frequency range. In our example it’s 5-21 GHZ. Then the IF bandwidth is important which in our case is DC to 5 GHZ. This means we can translate or convert any signal from 5-21GHZ, to 0-5GHZ, as a down-converter.
Conversion loss: it’s a very important spec and refers to the reduction in signal power from input port due to the mixing process. Lower losses are desirable.
L-R and L-I isolation: naturally we don’t want the LO to leak into the output. So the higher this isolation the better.
Level 15: our mixer is level 15. This means we need to provide a 15dBm signal to the LO port. There are other levels as well (e.g. 7 or 10)

3/7

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Mehdi

@MehdiHacks

Feb 23

🧵 RF basics: Attenuators.
A thread for beginners on:
1. What an attenuator does
2. Different types of attenuators
3. Understand the datasheet terms
4. Test its specs

1/4

An attenuator is a passive component that “attenuates” a signal’s amplitude, ideally not impacting its other parameters like frequency. Let’s say you have a 0dBm signal and connect it to an attenuator , and let’s say your attenuator is 20dB. In this case, you’re going to get a -20dBm signal after attenuation. It may not be exactly 20dB of attenuation, because attenuators like any other component or circuit, have some tolerance (let’s say +-0.5dB)
Most attenuators are bi-drectional, so it doesn’t matter which side of it you use as input or output.
Generally we can say we have 2 categories of attenuators: fixed and variable. Fixed is fixed: 1dB, 5dB, 20dB, etc. On a variable attenuator, you can change the amount of attenuation either manually (like physically with a knob/selector, as you can see with HP in the picture on the previous post) or programmatically/digitally (as seen in this diagram from Mini Circuits)

2/4

What are the most important specs of an attenuator?
1. Obviously the attenuation is the first: for a fixed attenuator, it would be just a number in dB (e.g. 20dB). For variable attenuator, it would be a range (e.g. 1-10dB, or 10-100dB)
2. Frequency range: the frequencies that you can attenuate the signal and expect the attenuator to do its job according to the datasheet (e.g. DC-6GHZ)
3. Power handling: how much power the attenuator can safely dissipate (e.g. 2W, 20W, …). Usually a high power attenuator is bigger and heavier because of heat sinks used.
4. VSWR: it’s a ratio describing impedance mismatch. A lower VSWR means better impedance matching , minimizing signal reflections.
Here’s a fixed attenuator from @MiniCircuits , and its datasheet

3/4

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Mehdi

@MehdiHacks

Feb 16

🧵 RF basics: amplifiers.
A short thread for beginners, on understanding an RF amplifier's datasheet specifications.

I've picked Mini Circuits ZX60-123LPN+, an ultra wide-band, low phase noise amplifier operating from 50MHZ to 10GHZ

1/5

What an amplifier does is in the name: it amplifies the signal (increasing the amplitude). Let’s say you have a -20dBm signal, and you need to make it 0dBm. So you use an amplifier that can add 20dB of gain at that frequency.
Amplifiers are active devices. They need external power to work, unlike passive components like filters, mixers and splitters.
For the sake of simplicity, we can say that amplifiers are opposite of attenuators (one amplifies, the other attenuates)

(Pictured is the inside of the amplifier chosen for this thread)

2/5

Let’s check the datasheet for our amplifier.
Broadband: this model is designed to work across a huge frequency span (from 50MHZ to 10GHZ). Not all amplifiers work this wide.
Gain, 16dB typ: This means typically (but not always) it provides 16dB of amplification. Some of these numbers in the datasheets have a min, typ and max value.
Gain flatness +-0.9dB, 0.05 to 6GHZ: this means that from 50MHZ to 6GHZ the gain doesn’t deviate by more than 0.9dB from the typical value.
Return loss, 20dB typ. 2GHZ: this means how much power is reflected back from the input or output port due to impedance mismatch. Here, manufacturer has used the best value, and that’s why they’ve picked 2GHZ. If you check the actual table, you’ll see that it’s not as great at other frequencies.
Low additive phase noise: I’ll write about phase noise in an upcoming thread. It’s important, and deserves a dedicated thread. Very short definition: phase noise is the random fluctuation in the phase of a signal, causing signal purity degradation (the lower the phase noise, the better)

3/5

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Mehdi

@MehdiHacks

Feb 1

🧵#temporary
I'm cleaning up my workshop.
Let me know if you want any of these (free. You only pay for shipping. Only EU. Or pick-up in Berlin)

1. Smart Car shield robot PCBs
2. PicoEMP PCB
3. Hackaday 2023 Berlin badge (a retro computer)
4. Lostik Lora device

1/4

2nd batch:

1. Adafruit Bluetooth LE sniffer (nRF51822)
2. Texas Instruments EZ430-CHRONOS hackable watch, with programmer and RF access board
3. AD9850 DDS signal generator module
4. Digilent Basys 2 FPGA board
5. TeensyConvolution SDR PCBs

2/4

Third batch :
4 LCD modules:
1. ILI9225 TFT (Arduino)
2. TS1620A 21 LCD 2x16
3. 3.2" SPI TFT (ILI9341), touch
4. Sipeed Maxi Go's LCD (2.4 inch TFT, capacitive touch screen 320*240)

3/4

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Mehdi

@MehdiHacks

Jan 31

🧵
Mega thread on RF, SDR, ham radio, and signal hacking:
I've been writing educational posts and threads on these topics.
To help finding them easier, I will put all the links here.
And I will link the new threads to the bottom of this meta thread every time I write one.

0/n

https://twitter.com/mehdihacks/status/1865460523875418250

1. How to build an RF lab on a budget

1/n

https://twitter.com/mehdihacks/status/1865460523875418250

https://twitter.com/mehdihacks/status/1865805626925875589

2. An intro to amateur radio

2/n

https://twitter.com/mehdihacks/status/1865805626925875589

Read 22 tweets

Mehdi

@MehdiHacks

Jan 10

🧵 SDR for beginners:
Receiving FM radio is the "hello world" of SDRs. It shows that the hardware/software setup is working.
Here I show 5 ways to do it, using an RTL-SDR dongle, increasing the complexity with each method:
GUI apps, cmd line, streaming, GNU Radio, and code.

1/6

1. Obviously the easiest way is to use a graphical SDR app. There are many:
Windows: HDSDR, SDR# and SDR-Console
Linux/macOS: gqrx, CubicSDR, and SigDigger
There are also vendor-specific apps (e.g. SDRConnect for SDRPlay)
Some apps can also be compiled from source.
This can't get easier: connect the SDR via USB port, connect a simple telescopic antenna to it, and tune to a known FM station frequency on your app and listen! (make sure you select the WFM demodulation)
Here's a screenshot from gqrx on macOS:

2/6

2. Next option is command line!
You can listen to the FM broadcast with RTL-SDR using this command:

rtl_fm -g 50 -f 93.1M -M fm -s 180k -E deemp | play -r 180k -t raw -e s -b 16 -c 1 -V1 - lowpass 16k

Basically, we use "rtl_fm" to receive and demodulate the samples, and then send audio data to "play" to actually play them.
Arguments used in rtl_fm:
-g : tuner gain settings
-f : the frequency to tune to
-M : modulation type
-s : sample rate
-E deemp : enables the de-emphasis FM filtering

Arguments used in play:
-r : sample rate
-t : input type
-e : sets the encoding to signed integer
-b : audio sample size in bits
-c : number of channels (1 means mono)
lowpass 16k: enables the lowpass effect to reduce noise

3/6

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