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Mehdi

@MehdiHacks

May 8

An introduction to Software Defined Radios.
A thread for beginners on:
1. What a SDR is
2. What you can do with SDRs
3. How it plays a role in the security/hacking world
4. How it’s used in ham radio.
5. How to choose/buy one
6. Link to more reading material

0/21

Disclaimer: I need to oversimplify many concepts, and also omit/skip some advanced ones. This is a huge topic that can’t be covered in a few posts, and my target audience is beginners.

What is SDR? If you’ve heard of RTL-SDR or HackRF but aren’t sure why they’re so popular among hackers, ham radio enthusiasts, and the SIGINT community, this thread is for you.
Traditionally, radios were fixed-function, and all implemented in hardware. An FM radio did just that: receive FM in the 88-108 MHz range. If you wanted to listen to let’s say shortwave (3-30MHZ), then you needed a new radio that had that functionality. You were limited to the hardware.
SDRs change this. With SDR, the hardware is controlled by software, giving you flexibility. Basically, many functions that traditionally were done in electronics circuits (e.g. filtering, demodulation, etc) are done digitally in SDRs.
Want to listen to FM? Easy. Want to explore Bluetooth or decode satellite downlink? Same device, just different software, or different settings in your SDR app. You define the functionality in software (e.g. which frequency to dial to, which modulation to use to demodulate the signals, what bandwidth to use to receive the signal, etc). Think of it like this: all the physical knobs that you had in the older radios, are now replaced with UI elements in software.

1/21

This brings up a world of possibilities: you could prototype systems in software, that were only possible to be made in hardware in the past. You can use the same SDR that is used to listen to FM radio, to sniff modern wireless protocols (e.g. Bluetooth). This hugely shortens the time to implement a new proof of concept for many use cases (production, research, idea validation, testing etc), and also saves you hardware cost, as most of your time would be spent in software (assuming that you’re using a commercial SDR, already built by another company, like what we cover in this thread)

2/21

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Mehdi

@MehdiHacks

May 2

🧵 RF basics: Power meter.
A short thread for beginners.
If you need to measure a radio signal’s power precisely, you need an RF power meter.
It basically shows the signal’s power in dBm (or milliwatts)
In this thread I will introduce the different types, with examples.
1/7

First of all, why do we need to measure a signal’s power with a power meter, if we can “see” the signal on the spectrum analyzer, which also shows the amplitude?
Well, there are many reasons. One of them is that spectrum analyzers are not as accurate as power meters when it comes to power measurements (we’re talking ~±1dB vs ~±0.2dB. This is important in some use cases like testing transmitters, regulatory compliance, etc). Power meters are also much more accurate for complex wideband signals (e.g. LTE). And, power meters/sensors can be calibrated against some standards. Not to mention the cost! For example a 26GHz spectrum analyzer is much more expensive than a 26GHz power sensor.
But why do we need to measure power at all? Apart from the above reasons, we need to make sure every component or circuit either receives the required power at its input, or generates output in the desired range, or both.
Although the unit for power is Watt (W), in the RF world it’s measured and described in dBm (dB relative to 1 milliwatt)

2/7

A simple definition, to avoid confusion: power sensors are the devices actually measuring the power. Power meter is the device that shows the measured value. Now, as you will see below, sometimes these 2 are sold as one integrated device, and sometimes they’re separate. (now that you know the distinction, I will use them interchangeably in the text)
Power meters/sensors can be categorized based on 3 features or parameters:
1. USB-based or stand-alone:
Some power meters are stand-alone (like the small immersionRC in the picture). It doesn’t need a computer to work. Some are USB-based, like the Anritsu shown in the picture. You need to use the accompanying software from the vendor.
2. Internal or external sensor:
Some power meters need an external power sensor to work. It’s like the power meter is the “interface” that can connect to different types of sensors. The 2 Agilent sensors you see in the picture, are sensors. They can only be connected to specific Agilent/Keysight power meters, and work with them.
Some power meters can work with both their internal sensor, and also accept external sensors (like the GenComm)
3. Average/CW vs Peak vs True RMS:
Depending on your use case or the signal you want to measure, you need to use the proper power sensor. To measure a simple unmodulated signal, you can simply use a CW/average sensor. An RMS sensor is better suited to measure complex or modulated signals. A Peak sensor can also measure the short bursts (which can’t be done by the other sensors), like pulsed RF or radar.
(we can also categorize based on the detection mechanism, like diodes or thermocouples, but I will skip that)
3/7

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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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