2 GHz Converter for the GBPPR Spectrum Analyzer
This is a RF converter which can be used to extend the receiving range of the GBPPR Spectrum Analyzer project or any other piece of RF test equipment. It consists of a Mini-Circuits SRA-11 mixer with a phase-locked Local Oscillator (LO) at 2 GHz. The SRA-11 mixer's RF and IF ports are padded with 3 dB attenuator pads to help all the mixer's port "see" 50 ohms, which reduces intermodulation products.
Any signal coming in the converter's RF port is mixed with the 2 GHz LO and the "new" signal is now on the IF port. For example, an incoming signal at 2.45 GHz would be mixed with the 2 GHz LO signal and appear on the IF port as a 450 MHz signal (and 4.45 GHz - but ignore that one). This "new" RF signal will be attenuated 6 dB by the two attenuator pads and another 7 dB or so by the conversion loss in the SRA-11 mixer. This new IF output frequency is now within the tuning range of the GBPPR Spectrum Analyzer.
This mixing process also works in "reverse." If you apply a 144 MHz signal to the converter's IF port, the new mixed frequencies on the RF port would be 2.144 GHz (2 GHz + 144 MHz) and 1.856 GHz (2 GHz - 144 MHz). Harmonics and sub-harmonics of these mixed frequencies will also exist, to a certain point. This is helpful for easily creating fairly high microwave frequencies.
The local oscillator of this circuit will be based around a Mini-Circuits ROS-2082-119 Voltage Controlled Oscillator (VCO). Other VCOs will work, provided they cover 2 GHz at around 2 to 3 tuning volts. The Phase-Locked Loop (PLL) will be based around a Motorola MC145151 parallel-programmed PLL synthesizer and a Fujitsu MB506 divide-by-256 prescaler. The reference frequency for the PLL will be from a 10 MHz Temperature Compensated Crystal Oscillator (TCXO). Using a TCXO is a bit of overkill, but in test equipment you don't want any frequency drift.
The Mini-Circuits SRA-11 mixer used in this project isn't ideal, but I had one available. The listed SRA-11 high frequency response is 2 GHz, but it works fine up to 3 GHz with just a slight increase in conversion loss (from 7 to 11 dB). RF power applied to either the mixer's RF or IF port should not exceed 0 dBm (1 mW).
Pictures & Construction Notes
2 GHz converter overview.
The 10 MHz Temperature Compensated Crystal Oscillator (TCXO) time base is on the left. It's an EG&G Part Number 1DN14-CV90-2201-1 (Model Number T424), and is from an old Qualcomm OmniTRACS control unit. This TCXO is covered in Issue #72.
The MC145151 PLL synthesizer is the 28-pin PLCC chip in the lower-middle. The raised pins provide a logic "1" for the PLL's N divider.
The Mini-Circuits ROS-2082-119 is in the upper-middle. Its RF output directly drives the LO port on the Mini-Circuits SRA-11 mixer on the upper-right.
The 8-pin IC just below the SRA-11 mixer is the Fujitsu MB506 divide-by-256 prescaler.
Closeup view of the PLL and VCO sections.
The capacitors in the PLL loop filter are polystyrene to minimize microphonics.
A LM140-5.0 provides the +5 VDC for the MC145151, MB506, and ROS-2082-119.
There is an optional tap on the MC145151's reference oscillator output (pin 26). This is handy for tuning the TCXO frequency or for providing a reference for another piece of test equipment.
Outside case overview.
It's mounted in an old California Amplifier MMDS downconverter case with a N connector for the RF port, a BNC connector for the IF port, and a F connector for the optional 10 MHz reference output.
Banana jacks are for the main +15 VDC input.