--Some easily added suggested improvements
The Time Machine, by Extended Spectrum Systems, is a simple direct conversion receiver kit that will provide quadrature (I/Q) signals suitable for use with a soundcard to permit I/Q reception with Linrad. Using quadrature reception doubles the bandwidth that can be processed and viewed in the waterfall and spectral display for a given soundcard sampling rate, compared with a conventional non-quadrature receiver. The Time Machine can process signals throughout the HF spectrum, and can be used as the second stage of a dual conversion receiver for example to receive 144 MHz or higher frequencies. This is how I use it with Linrad. I use the first IF of my homebrew receiver to mix down to about 10 MHz, and then The Time Machine takes over.
WhileThe Time Machine works very well for casual band tuning, the center frequency peak is a bit wider and more prominent than is ideal. Note that this center frequency peak is made up of several components. First, there is low frequency noise (60 cycle hum, ground loop noise, etc.) that is NOT due to The Time Machine. Secondly, there is a component that is due to low frequency noise entering the system within The Time Machine, that can be reduced or eliminated by changing several capacitor values in The Time Machine. Finally, the performance of The Time Machine can be improved and its frequency agility improved if the ICS-501 is removed from the circuit and the crystal oscillators replaced by an external oscillator. If this is done, attention to the phase noise performance of the external oscillator is of paramount importance if you are going to improve upon the performance of the original circuit! If you stick with crystal oscillators, replacing the ICS-501 PLL with a low-noise crystal oscillator circuit will improve the performance of The Time Machine.
The modifications shown here were suggested by Leif Asbrink, the creator of Linrad, after I sent him 'gif' files of the spectra I was obtaining with The Time Machine/Linrad combination. Leif recommended that capacitors C10 and C22 be changed to 100 uF, capacitors C19 and C21 be changed to 10 uF, capacitor C11 be changed to 470 uF, and capacitors C7 and C8 be increased to at least 470 uF. Expanded Spectrum Systems has officially adopted most of these changes and all Time Machine owners should have already received, or will receive shortly replacement capacitors for C8 and C11 (330 uF) and for C10, C15, C16, C17, C18, C19, C21, and C22 (100 uF). Future units will have these changes incorporated.
I added electrolytic capacitors of the appropriate values for C10, C22, C19, C21, and C11 by soldering miniature electrolytic capacitors in parallel with the existing capacitors, placing them on the underside of the circuit board. I used 1000 uF for C8, soldering it in parallel to the original capacitor on the component side of the board. I didn't increase C7 (yet). This produced a nice reduction in the width and height of the center frequency peak.
The second modification I did was to remove the ICS-501 from the circuit. This IC is a PLL clock multiplier, and adds some noise to the system. If you are building The Time Machine and will be using an external oscillator as I am, or building your own low noise crystal oscillator, then you can just not install the ICS-501. What I did was to lift the end of R1 attached to the ICS-501, and use it as the input point for my external oscillators. I just soldered an SMA female connector to the free end of R1. I grounded the input (pin 1) of the ICS-501, as otherwise it runs free and makes more noise. As my external oscillators produce sine waves, I run them through DJ8ES's buffer board, that was described in the 4th quarter issue of VHF Communications, to get the TTL signal for The Time Machine. I got the SMD board from DJ8ES, and the parts assembled easily. I am sure there are other solutions, but I had this board lying around and so I used it here. The folk at ESS will shortly have a daughter board that allows an easy SMA connection to The Time Machine when using an external LO. Note: as of early 2003, this board is now available. It is shown on the ESS website at http://www.expandedspectrumsystems.com/prod6.html I have used these daughter boards here at W3SZ with the PTS160 synthesizers as their LO input (directly, with no need for the DJ8ES buffer board), and they work GREAT!
What is the effect of these changes? The gif files tell the story. The center frequency noise is markedly attenuated by the capacitor changes. The modification to allow the use of external oscillators without the ICS-501 significantly increases the frequency agility of the unit, with no significant increase in noise levels vs when using the crystal oscillator.
Click here for the spectrum taken with the original "Time Machine" hardware and the original crystal LO at 10.125 MHz. Note the width of the center frequency noise peak
Click here for the spectrum taken with the unit containing capacitor modifications as noted above, again with the original crystal LO at 10.125 MHz. Note how much narrower the center frequency noise peak is with the modified unit.
The following spectra compare an original-hardware "Time Machine" with the original crystal oscillator LO to a modified "Time Machine" using a PTS computer-controlled synthesizer as the LO at the same frequency.
Click here for the spectrum taken with the original "Time Machine" hardware and the original crystal LO at 10.125 MHz. Note the width of the center frequency noise peak
Click here for the spectrum taken with the unit containing capacitor modifications as noted above, and using a PTS computer-controlled synthesizer as the LO, set for 10.125 MHz. Again, note how much narrower the center frequency noise peak is with the modified unit.
Click here to go to my page describing how I used The Time Machine and Linrad for the 2002 ARRL Frequency Measuring Test.
Copyright 1997-2007 COPYRIGHT Roger Rehr W3SZ. All Rights Reserved
