Chris Hornbeck wrote:> On Sun, 17 Apr 2005 15:55:23 -0400, robert bristow-johnson > <rbj@audioimagination.com> wrote: > > >>Heathkit "Hot Water" HW-100, the cheapest 5-band SSB a kid could afford). >>although there were a lot of tubes, and 2 or 3 transistors in the VFO, there >>were no all-pass filters nor direct Hilbert transformers in that box. the >>SSB was generated the old fashioned way with AM (at some IF frequency that i >>forget) and an nasty sharp crystal-lattice filter that was 2.1 kHz wide. > > > Wouldn't those have had to be DSB modulators, and to have just > stripped the unwanted sideband? Wouldn't the carrier from an > AM modulator have been too large as a filter residual? > 'Course, times have changed! > > Chris Hornbeck > 6x9=42 April 29The residual was 40 t0 60 dB down. We used balanced modulators to generate a double-sideband suppressed-carrier signal with about a 500 Hz gap between upper and lower sidebands. After filtering out one sideband, there was your signal. Without carrier, you need linear amplifiers in subsequent stages. (Only a synchronous demodulator could recover the signal while both sidebands were present.) Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Hilbert transform question
Started by ●April 17, 2005
Reply by ●April 18, 20052005-04-18
Reply by ●April 18, 20052005-04-18
Chris Hornbeck wrote:> On Mon, 18 Apr 2005 00:37:54 -0400, robert bristow-johnson > <rbj@audioimagination.com> wrote: > > >> it would still need a good and >>sharp filter. i remember that the crystal lattice filter was a component >>mounted off of the circuit boards and 2 cables connected to it. > > > And in those days the filter was PFM ( pure ... magic ) although some > intrepid souls made their own, largely by (literally) cut-and-try, > by filing on quartz crystals and inserting them into some hoped-for > band pass filter. > > In that era the Collins LC (? is my memory correct?) filters at > 455 kHz were considered to be "The Shit" because they had some > really deep slopes. > > Probably nobody worried too much about residual carrier anyway; > a small beat somewhere wasn't the end of the world. > > Thanks, > > Chris Hornbeck > 6x9=42 April 29Tuning a crystal up is easy: just thin it a little with fine emery paper. Down was thought to be impossible by some, but I once tuned a crystal down by writing "down" on it with a pencil, much to the amazement of my buddy who thought it was a joke. Of course, any scribble would have worked. The extra weight of the "lead" changed the frequency. We used 5 MHz crystals exposed to the vapor stream to measure film thickness when evaporating metals and insulators in a vacuum system. The change in frequency is a measure of the deposited mass. The sensitivity can be changed by varying the size of the hole open to the vapor. We converted frequency shift to voltage, differentiated that, and closed a loop around the heater current to maintain constant deposition rate. All analog: in 1964, there was no other practical way. Believe me: digital is simpler, once you get the hang of it. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 19, 20052005-04-19
Chris Hornbeck wrote:> On Sun, 17 Apr 2005 15:55:23 -0400, robert bristow-johnson > <rbj@audioimagination.com> wrote: > > >>Heathkit "Hot Water" HW-100, the cheapest 5-band SSB a kid could afford). >>although there were a lot of tubes, and 2 or 3 transistors in the VFO, there >>were no all-pass filters nor direct Hilbert transformers in that box. the >>SSB was generated the old fashioned way with AM (at some IF frequency that i >>forget) and an nasty sharp crystal-lattice filter that was 2.1 kHz wide. > > > Wouldn't those have had to be DSB modulators, and to have just > stripped the unwanted sideband? Wouldn't the carrier from an > AM modulator have been too large as a filter residual? > 'Course, times have changed! >The SB-102 is supposed to be the same circuit as the HW-100 but with a better oscillator and fancier case. It had a balanced modulator with carrier suppression, as does just about any other SSB scheme I've seen. In my '50s and '60s ARRL handbooks the phasing method and the filter method (ala HW-100) ran neck and neck. The filter method wins out for production radios because crystal filters are hard to engineer but (relatively) easy to build once you get the recipe right. The phasing method was (and is) often easier for home brew because it didn't take as much engineering but _did_ take more fussing. Nowadays you take have I-Q demodulators straight from the GHz down to baseband and into a DSP -- and if you want to demodulate SSB it'd better be with phasing! -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●April 19, 20052005-04-19
Reply by ●April 19, 20052005-04-19
Tim Wescott wrote:> Chris Hornbeck wrote: > > > On Sun, 17 Apr 2005 15:55:23 -0400, robert bristow-johnson > > <rbj@audioimagination.com> wrote: > > > > > >>Heathkit "Hot Water" HW-100, the cheapest 5-band SSB a kid couldafford).> >>although there were a lot of tubes, and 2 or 3 transistors in theVFO, there> >>were no all-pass filters nor direct Hilbert transformers in thatbox. the> >>SSB was generated the old fashioned way with AM (at some IFfrequency that i> >>forget) and an nasty sharp crystal-lattice filter that was 2.1 kHzwide.> > > > > > Wouldn't those have had to be DSB modulators, and to have just > > stripped the unwanted sideband? Wouldn't the carrier from an > > AM modulator have been too large as a filter residual? > > 'Course, times have changed! > > > The SB-102 is supposed to be the same circuit as the HW-100 but witha> better oscillator and fancier case. It had a balanced modulator with> carrier suppression, as does just about any other SSB scheme I'veseen.> > In my '50s and '60s ARRL handbooks the phasing method and the filter > method (ala HW-100) ran neck and neck. The filter method wins outfor> production radios because crystal filters are hard to engineer but > (relatively) easy to build once you get the recipe right. Thephasing> method was (and is) often easier for home brew because it didn't takeas> much engineering but _did_ take more fussing. > > Nowadays you take have I-Q demodulators straight from the GHz down to> baseband and into a DSP -- and if you want to demodulate SSB it'dbetter> be with phasing! > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.comYes, but watch out for DC offsets! It is often better to mix down to an IF that can be sampled by a two channel ADC and perform the final downconversion in DSP. John
Reply by ●April 19, 20052005-04-19
On Mon, 18 Apr 2005 10:08:50 -0400, Jerry Avins <jya@ieee.org> wrote:>We used 5 MHz crystals exposed to the vapor stream to measure film >thickness when evaporating metals and insulators in a vacuum system. The >change in frequency is a measure of the deposited mass. The sensitivity >can be changed by varying the size of the hole open to the vapor. We >converted frequency shift to voltage, differentiated that, and closed a >loop around the heater current to maintain constant deposition rate.Dude! I hope you didn't have to calibrate this, but there's one thing to be said about the wonderful digital clean world of the future: it ain't here yet. And I'm astonished at such an elegant solution. Hats, gentlemen, Chris Hornbeck "Hum is more than just not knowing the words." -ha
Reply by ●April 20, 20052005-04-20
Chris Hornbeck wrote:> On Mon, 18 Apr 2005 10:08:50 -0400, Jerry Avins <jya@ieee.org> wrote: > > >>We used 5 MHz crystals exposed to the vapor stream to measure film >>thickness when evaporating metals and insulators in a vacuum system. The >>change in frequency is a measure of the deposited mass. The sensitivity >>can be changed by varying the size of the hole open to the vapor. We >>converted frequency shift to voltage, differentiated that, and closed a >>loop around the heater current to maintain constant deposition rate. > > > Dude! I hope you didn't have to calibrate this, but there's one thing > to be said about the wonderful digital clean world of the future: it > ain't here yet. > > And I'm astonished at such an elegant solution. Hats, gentlemen,Calibration isn't difficult. For each material, one test sample. (It's established that delta-f is linear with mass, hence thickness. The evaporation is sustained until the frequency is shifted some standard amount -- approximate, but the exact value recorded -- and the film thickness measured interferometrically. (A Fabry-Perot interferometer is simple: a semitransparent reflecting film on a glass slide, laid over the step edge of the evaporated film ans illuminated with collimated light of known wavelength. The displacement of the interference fringes gives the height in half-wave steps, with quarter-wave or better easily estimated.) I have the means to measure the film thickness at home. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
