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Electronics => RF, Microwave, Ham Radio => Topic started by: DannyCoates on January 11, 2024, 03:41:11 pm

Title: Low frequency downconverter
Post by: DannyCoates on January 11, 2024, 03:41:11 pm
Hello, I'm trying to create a simple downconverter for a relatively low carrier frequency of 10MHz and IF of about 500kHz. Aiming for minimal number of parts and maximum integration. Trying to avoid designing a discrete mixer because it will inevitably take up more board space. Also trying to avoid an IC so general-purpose that it ends up taking more space than a discrete mixer.

Looking at modern mixer IC's most of them have a minimum frequency. Please can anyone tell me why there's a lower limit and if they would work with lower frequencies regardless? I don't think it's because they have internal tuning because most of them require an external matching network.
Title: Re: Low frequency downconverter
Post by: PA0PBZ on January 11, 2024, 03:48:53 pm
Did you think about something like an SBL-1? RF and LO starts at 1MHz, IF from zero. You just need a bit more LO juice to make it go.
Title: Re: Low frequency downconverter
Post by: geggi1 on January 11, 2024, 09:25:36 pm
You can use one of these circuits.
https://www.qsl.net/n9zia/vlf/index.html (https://www.qsl.net/n9zia/vlf/index.html)
https://www.giangrandi.org/electronics/lwupconv/lwupconv.shtml (https://www.giangrandi.org/electronics/lwupconv/lwupconv.shtml)
I have seen variations of vlf converters where the If port is used for the low frequency.
You might see some loss in port isolation and require som extra filtering.
Title: Re: Low frequency downconverter
Post by: Solder_Junkie on January 21, 2024, 09:55:51 am
Use a low noise crystal oscillator, such as the ones used in this BFO circuit, amplify the oscillator level to +10 dBm and use a 3 dB attenuator to feed it into a level 7 diode ring mixer. The oscillator circuit is based on one suggested by Dr Ulrich Rohde (of Rohde and Schwarz).
https://www.qsl.net/g4aon/pdfs/BFO%20v2_3.pdf (https://www.qsl.net/g4aon/pdfs/BFO%20v2_3.pdf)

Diode ring mixers need to be terminated in 50 Ohms on all ports, otherwise the isolation degrades.

A 50 Ohm input low pass filter can be designed using the free program "Elsie" http://tonnesoftware.com/elsie.html (http://tonnesoftware.com/elsie.html) use sensible values for the Q of the inductors and capacitors, try 200 to 500.

SJ