Quote:
Originally Posted by G0HZU_JMR
The single diode limiter (with a bias choke across it) probably isn't a good choice as a protection limiter. It won't offer very good isolation for large signals. A better limiter design will use four diodes arranged as a classic clipping limiter.
A typical silicon diode will have a forward voltage of about 0.8 to 0.9V when biased hard so the clipping limiter will produce a rounded square wave output at about 1.8Vpkpk when hard limiting. Hiding inside this 1.8Vpkpk rounded square wave will be a fundamental sine wave of about 2.3Vpkpk (or 1.15Vpk)
This will produce a through leakage power of (1.15^2)/100 watts into a 50R load when hard limiting. This is about +11dBm at the fundamental frequency. That's what you can expect from a classic 4 diode limiter. Some limiters only use two diodes, and this can help extend the bandwidth to >2GHz. The 2-diode limiter won't be as rugged though.
I'm a bit out of date when it comes to the capability of modern limiter diodes, but 15-30 years ago a well-designed silicon (4) diode limiter could withstand a 5-10W incident power (without damage) whilst limiting the leakage to less than about +13dBm. The limiter should be transparent to signals that have a maximum amplitude that is in the order of <0.6Vpkpk. i.e. the limiter should not cause any significant distortion to waveforms smaller than this.
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Thanks. The 4 rectifier diode idea is still in my mind but I am concerned about the large diode junction capacitance which may call for a matching network to tweak the S11 and S21 at high frequencies.
I have taken another potshot at the PIN diode. I ordered a Skyworks SMP1330-005LF last night. It is a PIN-Schottky pair, having a threshold about +10dbm, C=0.7pF, 14db attenuation at +30dbm working up to 4GHz (see power characteristics attached). I speculated that the cheapo pin-limiter does not work well because it uses only a single diode PIN diode.
https://www.skyworksinc.com/-/media/...es_200050N.pdf
I am going to add two 1kV 1206 10nF and 250mA SMT fuse plus a 70V 4532 SMD ceramic GDT (capacitance = 0.5pF) to safeguard against "catastrophic event". I will house them in RF shield instead of using a metal box.
I have been looking at the integrated SKY1662-632LF datasheet:
https://www.skyworksinc.com/-/media/...LF_202945L.pdf
It is kind of interesting to see how they put a simple series LC network to optimize the S11 and S21. With L = 1.8nH, C=15pF for 2.45GHz, they transpose the peak and trough for S11 and S21 to the right. The equivalent circuit of a PIN diode (in open state) is like a small inductor in series with the paralleled capacitor and a very large resistor. The S21 and S11 look a bit like a very broadband bandpass filter.
PS: I have received the ATC caps and other goodies that you sent. Thank you so much.
I am building a "Q detector probe" with your ATC 1pF caps. It will use FET MMBF310 and MMBF3906 as impedance convertor. The device will be used to find approx Q and resonance frequency of air variable tuning gangs and IF transformers for MW, SW and FM radios in unpowered state.
The ATC100 series caps have Q about 10,000..quite badass!