Thanks guys, time to start building.
Dale
WB4LIP
On Thursday, October 16, 2014, Steve VK2SJA <vk2sja@xxxxxxxxxxx
<mailto:vk2sja@xxxxxxxxxxx>> wrote:
>Is there a link somewhere to your construction guide?
minima_main_page | ae5phradionotebook
<https://sites.google.com/site/ae5phradionotebook/minima_main_page>
Down towards the bottom of the page.
>
> WB4LIP
>
> On Thu, Oct 16, 2014 at 12:10 AM, Mac A. Cody <maccody@xxxxxxx
<javascript:;>> wrote:
>
>> Steve,
>>
>> Sorry about the misunderstanding. I had forgotten that the
>> circuitry of the bidirectional amp was not symmetrical.
>>
>> Glad you like my construction guide. I've taken a bit of
>> a 'breather' from working on it due to time, other projects,
>> waiting for design issues to sort out, and a bit of personal
>> 'burnout' on the Minima project. The notes that you are
>> working on is a great idea.
>>
>> 73,
>>
>> Mac AE5PH
>>
>>
>> On 10/14/2014 10:03 PM, Steve VK2SJA wrote:
>>
>>> Thanks Mac,
>>>
>>> This is Joe's own suggestion for the TX side of the
bidirectional amp.
>>> Joe
>>> and I were talking about the other side/direction, the RX side
of the
>>> amp.
>>>
>>> I think Joe was saying he thought that he may have seen
somebody else's
>>> suggested mod to reduce current here? At least that's how I
was reading
>>> it.
>>>
>>> Great "Construction Guide" BTW.
>>>
>>> 73, Steve.
>>>
>>>
>>>
>>> Steve,
>>>>
>>>> I've copied Joe Rocci's original post below.
>>>>
>>>> TL'DR:
>>>> Change the 220 Ohm collector resistor to 1000 Ohms to decrease
>>>> the current consumption. Increase the emitter degeneration
>>>> resistor to 20 Ohms to bump the gain up to 20 dB.
>>>>
>>>> 73,
>>>>
>>>> Mac AE5PH
>>>>
>>>> Original message posted by Joe Rocci:
>>>> Hi Steve
>>>>
>>>> Here are some results from modeling the Tx section in LTSpice:
>>>>
>>>> 1) Gain is about 17db @ 20 mhz
>>>> 2) Input match (return loss) is about 16db and pretty flat up
to 50
>>>> Mhz
>>>> 3) Output match is quite good, at the expense of about 6db
gain loss,
>>>> due to the series output matching resistor
>>>> 4) Current draw is about 55ma
>>>>
>>>> Some comments:
>>>> 1) The voltage-amp stage is biased at almost 11 volts on the
>>>> collector,
>>>> leaving very little headroom for large voltage swings.
>>>> 2) The emitter follower buffer stages draw the lion's share
of the DC
>>>> current, but are non-optimum for driving large voltage swings
into
>>>> non-resistive loads at high frequencies; that's why so much
static DC
>>>> current is required.
>>>> 3) If you increase the 220 ohm collector resistor to 1000
ohms, the
>>>> collector drops closer to 5.5 V, and the overall current
consumption
>>>> drops to about 28 ma. If you then increase the emitter
degeneration
>>>> resistor to 20 ohms, you end up with about 20 dB gain (@20
Mhz), 22 dB
>>>> input match, and presumably much better output voltage swing
headroom.
>>>> I
>>>> discovered all this while trying to goose the amp up for a little
>>>> better
>>>> performance at 30 Mhz, anticipating a new crystal filter.
>>>>
>>>> Of course, all of this is 'on paper' and needs to be verified.
>>>>
>>>> Joe
>>>> W3JDR
>>>>
>>>>
>>>> On 10/14/2014 08:22 PM, Steve VK2SJA wrote:
>>>>
>>>>> Thanks Joe,
>>>>>
>>>>> So ~16 to 20dB in both directions.
>>>>>
>>>>> I had a quick look around the archive this morning. If there
is a mod
>>>>> for
>>>>> reducing current in the RX side of the bidirectional amp
I've managed
>>>>> to
>>>>> miss it.
>>>>>
>>>>> Anyone know of such a mod?
>>>>>
>>>>> The original W7ZOI bidirectional amp article quotes a noise
figure of
>>>>> 5.8dB when the amp was setup for about 16dB of gain. So
4.5dB seems
>>>>> an
>>>>> improvement on that and around the same ball park. So the
LTSpice
>>>>> simulation is probably on the money.
>>>>>
>>>>> Last night I was reading EMRFD and learnt how to go about
measuring
>>>>> noise
>>>>> properly. I've ordered a LNA and when it arrives I'll try
measure it
>>>>> in
>>>>> the real-world and report back.
>>>>>
>>>>> I've been reading that at HF frequencies there is so much
atmospheric
>>>>> noise that a 3kHz wide SSB rig will typically see about 14dB of
>>>>> noise.
>>>>> And
>>>>> that as such any amp with a noise factor (NF) of of a bit
less than
>>>>> ~14dB
>>>>> is probably adequate for this application. Because you won't
see the
>>>>> locally generated amp noise over the atmospheric noise
anyway. So
>>>>> while
>>>>> we
>>>>> should always design and build as best we can. From a
practical stand
>>>>> point is there anything to be gained by reducing the NF to a
figure
>>>>> much
>>>>> below ~6dB?
>>>>>
>>>>> 73, Steve.
>>>>>
>>>>>
>>>>>
>>>>> Steve
>>>>>>
>>>>>> LTSpice simulation indicates about 16db gain at 20 mhz with the
>>>>>> component
>>>>>> values shown on the web page. The standing current is also very
>>>>>> high,
>>>>>> at
>>>>>> over 40ma. I think there was a mod to bring the current
down? Also,
>>>>>> the
>>>>>> simulated noise figure is about 4.5db, pretty high.
However, I've
>>>>>> never
>>>>>> checked to see how accurate LTSpice is in this regard.
>>>>>>
>>>>>> Joe
>>>>>>
>>>>>> ----- Original Message -----
>>>>>> From: "Steve VK2SJA" <vk2sja@xxxxxxxxxxx <javascript:;>>
>>>>>> To: <minima@xxxxxxxxxxxxx <javascript:;>>
>>>>>> Sent: Tuesday, October 14, 2014 6:40 AM
>>>>>> Subject: [minima] RX gain of Bidirectional Amp?
>>>>>>
>>>>>>
>>>>>> Hi Guys,
>>>>>>>
>>>>>>> The gain of the Minima Bidirectional Amp in the TX
direction is ~16
>>>>>>> to
>>>>>>> 20dB.
>>>>>>>
>>>>>>> Can someone tell me what the gain is supposed to be for
the other
>>>>>>> half
>>>>>>> of
>>>>>>> the duel Amp, in the RX direction? Same?
>>>>>>>
>>>>>>> 73, Steve.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>>
>>
>>
>
